Many new players try Soraka once (particularly because her IP cost is so low), find out that she cannot possibly pull off any form of DPS/Carry and that she isn't the world's most solid tank with the suggested build, get frustrated, lose that game, and never play her again. It doesn't help that there is a distinct lack of good guides for her.
So let me get a few things out of the way:
Soraka is NOT:
A carry capable of eliminating enemies 1vX lategame
An effective ganker. She lacks a "hard" disable/initiate (Stun/Taunt/Fear/Knocked Airborne/etc), having only the "soft" disable of Silence.
A great solo hero
Recommended if you want to play the carry for your team
Capable of keeping herself and an allied champion in-lane for almost the entire early game, providing you with experience/gold gain about equivalent to being solo
An amazing asset to her team late game with the right item build
One of the best mid-game lane pushers
Capable of escaping most solo champions intact and allowing an ally to come out victorious if 2 champions try to gank the two of you
Recommended if you want to greatly aid your team but aren't the world's (or your team's) best solo mid carry player.
Passive: Consecration - Increases surrounding allies Magic Resistance by 16. Not the world's best passive, but it provides a noticeable boost to your team for the entire game and is handy to have around.
Q: Starcall - A shower of stars falls from the sky, striking all nearby enemies for magic damage and reducing their magic resistance temporarily.
Strikes nearby enemy units for 60 / 90 / 120 / 150 / 180 (+.25 per ability power) magic damage and reduces their Magic Resistance by 8 for 8 seconds This effect stacks up to 20 times.
42 / 54 / 66 / 78 / 90 Mana
Cheap, low cooldown, spammable, and long-range, this ability is handy for very early harassment, last hits, and the debuff in team fights, as well as constant procs of on-cast items. You really only need one level of it for most of the game, however, as the damage does not increase by that much and your role beyond early laning is not to deal out the damage. Come later-game, this puts you far too much into the line of fire to be used effectively as damage, but still spam it whenever you have an enemy in your sights for the debuff.
W: Astral Blessing -Soraka blesses a friendly unit, restoring health and increasing armor for a short time.
Restores 80/135/190/245/300 (+1) health and grants 35 bonus armor for 9 seconds.
Handy. Keeps your life bar and your ally's life bar topped off, both proactively through armor, and retroactively by healing them. Make sure your lanemate knows when you have this, as it can save them a lot of time that would normally be spent going back to base. Of note is the fact that this ability also has a 1:1 AP scaling, so even when it caps out on level you can continue to raise its effectiveness by continuing to get AP. 55 AP is the same increase as one level of the skill. This effectively makes it so you cannot be harassed early, and later game giving someone an extra 300-500 HP can mean the difference between a death and a kill.
E: Infuse - If cast on an allied champion, restores mana. If cast on an enemy champion, silences them for a few seconds.
If cast on an allied champion, restores 50 / 100 / 150 / 200 / 250 Mana.
If cast on an enemy champion, silences them for 1/1.5/2/2.5/3 second(s) and deals 50/100/150/200/250 (+0.6) magic damage.
Probably the most subtly Overpowred ability in the game. Go ahead, re-read it. For free, it allows you to restore your own mana, restore your lanemate's mana, damage at a long range, or take a caster out of the fight for several seconds. For free. Early game, this is like having an extra 17mp/5 per level that you can divide between yourself and your lanemate at will or potentially harass with for free. Lategame, when you're rocking 40% cooldown reduction, 5th level is the equivalent of 139mp/5 if you use it on yourself/an ally, and can completely shut down an enemy Ryze/Fiddle/Morgana/etc in a team fight, as well as score some pretty easy last hits on fleeing champs. One thing to keep in mind is the cooldown. Do not throw this out willy-nilly in a teamfight, as you will find yourself wishing you had saved it in 4 seconds when you're 50 mana short of a Wish, Kat/Nunu is channeling in your team, Ryze just went Super Saiyin, or someone runs past you at 200 HP.
R: Wish - Soraka fills her allies with hope, instantly restoring health to all friendly champions.
Instantly restores 250 / 400 / 550 (+2.0 per ability power) health to all allied champions.
250 / 350 / 450 Mana
The cost is huge, especially at the higher levels, but the result can be pretty amazing. 550 HP is generally greater than a quarter of a champion's health bar, meaning if you can use this while your team is suffering AOE damage, you effectively get a free champion out of it (as 4 other champs x 25% of their HP = 100% of a champion). Add in the fact that it gets 2:1 AP ratio and the low cooldown at level 3 (especially with CDR), and lategame you can be throwing out 800-1000 HP heals to every member of your team every 40-60 seconds.
5. E3 (or W2 if you're being harassed hard)
7. W2 (or E3, whichever you didn't take at 5)
9. E5 (Maxed)
13. W5 (Maxed)
16. R3 (Maxed)
18. Q5 (Maxed)
Always take Rally. Gives an AOE heal and damage boost, greatly helping your team. Someone on your team needs this, and since you are already a healer, this is a good choice.
For the second spell, you have a few options.
Fortify. Having one of these on a team discourages tower diving and can save a number of towers over the course of the game. However, it's only really worthwhile if you have the mastery, so it depends which mastery setup you use.
Clairvoyance. Again, invaluable for a team to have, so you may as well be the one to carry it, assuming you can take the mastery.
Heal. If you really want to pump out the healing, having 4 sources beats having 3 sources.
Clarity. Not really recommended, as you should not be having mana issues unless you are being wasteful with Starfall (only use it to last hit!) due to Infuse. Can be handy to not have to worry about keeping 450 mana on hand for Wish level 3, however.
Flash. Get out of jail free. There's really no one it's not good on.
Exhaust. You lack a movement affecting crowd control, so this can provide it. This + Infuse is basically a stun.
Teleport. If you're going push-Soraka (rush Aegis), this can be handy to take advantage of creep waves to knock towers down, but someone like Sivir is better at that.
Really I'd suggest one of the first two depending on your setup. Never take Ignite (you don't need the kills), Revive (I shouldn't need to explain this one), Ghost (flash does what it does better), Cleanse (again, either flash will get you out or you were dying anyways, even with cleanse)
Suggested Runes + Masteries:
Tanky Soraka - 9/21/0 Masteries (get CDR, AP, and Spell Pen in the 9, and HP, %AP, HP regen based on MP, Dodge + Mobility, and MR in the 21). Flat HP Quints, HP Marks, MR Glyphs, MP/5 Seals. You're going to want an early Spirit Visage to make up for the lack of CDR. Fortify is your secondary spell of choice. 0/21/9 also works here for more Regen, but I'd suggest getting at least 3% CDR from your Glyphs in that case.
Handy Soraka - 9/0/21 (get CDR, AP, and Spell pen in the 9, and Exp, Regen, Mana Regen, CDR, movespeed, in the 21). Flat HP Quints, MPen Marks, CDR Glyphs, MP/5 Seals. More spammable spells, less survivability. Clairvoyance is your secondary spell of choice.
Suggested Item Build:
<Work in Progress>
Testing the following items on Soraka - Archangel's Staff, Abyssal Scepter, Frozen Heart, Soul Shroud, Aegis of the Legion, Guinsoo's Rageblade, Spirit Visage, Stark's Fervor, Innervating Locket, Boots of Mobility, Mercury Treads, Banshee's Veil.
In general, Auras, MP Regen, AP, Survivability, Cooldown reduction.
Remember, and I cannot stress this enough, that communication is key to playing Soraka. Before the game even starts, you should talk to your team and make sure you end up laning with a Carry or a Mage, preferably someone with mana needs. Make sure they know which aura items you are going and when so they don't get overlapping ones. Make sure they know that rather than having to port back, you can refill their health and mana bars. Make sure they know that they can call out for Wish later if their HP gets too low (also make sure to watch the bars on the left yourself. If you ever see 3 or 4 people who could benefit fully from wish, just cast it). Being a premade on vent is immensely beneficial, but even Pubbing you can be a great asset to your team.
Your ideal lane partner is someone squishy with high DPS who normally has trouble staying in a lane early but ends up very powerful if they can. DPS Mages and Carries are your best choices. Mages because they benefit from your -MR from starfall as well as your mana/-CD, and Carries because their endgame power tends to be directly proportional to how long they can stay in lane getting last hits. Failing that, try to get paired with Rammus or Malphite, who can take advantage of the +Armor your W and Aegis will grant for extra damage. Your ideal lane enemy is another mage, since the +16 MR aura will make their job tougher from the start, Infuse can silence to shut them down, and squishier characters can be easily forced out of lane or killed early game by starfall and infuse. Squishy physical champions are not bad as well, as your armor buff from Blessing helps to shut them down and they are also prone to being forced out or killed early. You don't really want to be up against tanks, as you will have a harder time harassing them and their power lies in disables largely more than damage, which makes your passive and armor buff less useful.
Try to stay near allies, as you are not very poweful on your own, and your power rather lies in making someone else their best. Once you have a few items under your belt, you can generally hold you own, harassing with Infuse/Starcall and refilling your own health with Wish. Try to harass down those enemies without HP regen.
When groupfights start breaking out, you have two choices. If you are not very farmed yet, go push a lane with an outer turret, making sure to keep an eye on allies and fire off your wish to save them. If you do have multiple aura items, make sure you stay with your group, granting them all the benefit of your auras, firing off your Blessing heal on whoever they seem to be focusing on, disabling their mage with Infuse, and spamming Starfall whenever it comes up to soften up your enemies for your mages.
If someone tries to gank you, run towards the nearest turret, pausing only to spam your spells. Make sure to do this, especially Astral Blessing to keep your HP and Armor up. But hitting them with Starfall and Infuse can be just as important. I've turned around countless ganks because they've been so determined to kill me that they ignore the fact that every few seconds they're losing a chunk of their life to starfall, and they underestimate the effect that Astral Blessing has on my life total. The only times you will really be in trouble are if you are hit by multiple heavy crowd control effects (Blitzcrank, Ashe/Warwick's Ult + other CC, Alistar's one-two, etc), especially close to their Turret.
When pushing a tower with a Cannon Minion, throw Astral on it as soon as the turret focuses it and hits it a few times. This will top off its HP and increase its effective HP by 30%, giving you and your minions longer to take the turret down. Later on, if you have Locket, make sure to spam your spells to keep locket up, keeping your creeps fully healed.
Make sure to save your Infuse in teamfights. It is not a nuke, it is not an initiation/harassment ability, it should be saved to turn things around, especially if they have a channel-dependent champion on their team.
I hope this has helped enlighten you on how to play Soraka without ragequitting. I'm open to any and all suggestions as well, as this is my first attempt at a guide.