this is for all those newbies who are overwhelmed with the store in the game and dont know what to buy and when to buy it. This will be a general guide on rules of thumb when it comes to purchasing items
a good rule of thumb is to always buy something whenever your by the store, sure you can buy the finished item if you have enough gold, but dont save up for it, instead buy each peice of the item if you have the space. the more items you have, the more efficent you can be and this will most likly help you earn gold faster to get that final item.
when you start a match, you start off with about 500 gold or so, this is enough to buy 1 or 2 items and a few potions. usually, you'll want to start off with a regen item, either health or mana. This is a fast pace game and you'll soon learn that harassment is a big factor in the game. getting a health regen item will help counter this harassment. if your hero has an ability to heal, get a mana regen item instead so that you can use this heal whenever you need it, and also allows you to be more flexible with your mana. the only time you'll want to start off with either a stat booster item (damage, health, mana, crit) is when your ranged and confident that you can lane/harass/farm without taking much damage.
the next item you should get are Boots, first you must buy the boots of travel, this is the base item for all boots. then you must think what hero type are you and get your boots depending on that. your choice of boots should really depend on the champions you'll face against. if your gonna be against many disablers and ensnares, getting mercury treads is always a good choice. if your going against champions with a natural movement speed boost, boots of mobility can help close the gap. but no matter what, you want to get one of these boots. the faster you can move, the faster you can do damage. if you cant catch up to your opponent, you cant do damage to them. and if you cant out run your opponent, chances are your as good as dead.
always plan ahead, figure out what your champion's role is and what playstyle your going to be taking. a good rule of thumb is to first buy items that cover what they are weak in. for example, if your hero naturally has good attack speed, look into getting more damage, if your hero requires you to attack faster (such as taric's mana regen from attacks and his imbune's cooldown reduced by his attacks) get attack speed boosters. many champions have abilities which can scale with different things, corkie's gatling gun scales with your attack damage while his other abilities scale with ability power. if this is the case always pick one path to go down and stick with it. for example with corki, you can either focus on being an AP user and boost his abilities, if you do, stick with AP items and not worry about improving his gatling damage, if your going attack power, do the vice versa.
when you've decided what items to get, now you'll have to decide when you wanna get them, when you'll have to return to base to buy em. usually dont go back to base if your doing well in your lane. the more time spent away from the battlefield, the less experience and money you'll earn. getting levels in this game is very important, since it allows you to have stronger abilities and higher stats. this game, you can easily get 3 or 4 levels above your enemy if they spend too much time away from the battle. but there are items which you would want to get as soon as possible, those are the items that improve over time. for example, rod of ages, this item improves every min and takes 15 mins to get it's full stats. goddess tears, this item increases your mana pool by 4 every time you cast a spell. soulstealer, this item gives you 10 ap for each champion kill and 5 for assists. warmonger, increase your health and health regen for each kill you do, even more for champion kills. Bloodthirster is another item that improves with kills, but this also resets it's bonuses whenever you die, so make sure your survivability is there before purchasing this weapon.
another item type you'd wanna buy asap are gold items, these items gives you a certain amount of gold per 5 seconds. the sooner you buy them, the sooner they end up paying for themselfs and the rest becomes pure profit. but then these items tend to be weak and cheap so only get them at the beginning of the game, later on you'll probably want to sell them for a better item once you've got the gold.
getting lifesteal is a great way to up your survivability, but this means that you must be attacking in order to regen your health. if you find yourself getting harassed a lot and not able to get into the fight much, dont bother getting life steal until later on in the game. instead get a health regen item. life steal items should generally be on charaters who have extremely high damage (crits included) or extremely high attack speed, or better yet, a combination of both. since this item calculates off the damage you've done on the enemy (armor is taken into account)
for champions who have the ability to cast abilities quickly, getting sheen is usually a must. this item gives you a boost of 100% of your base attack power every time you use an ability (3 second internal cooldown). if your champion is able to cast an ability every 3 seconds, this would end up being the highest physical damage boosting item in the game. usually a hero's base attack is about 70-90 by the time they get this item. the amount of damage it adds compared to the cost is the highest in the game. but it doesnt end there. you can upgrade the sheen into either a lichbane or a triforce and it should always be upgraded to either one of these. the triforce will add many other stats as well as give your attacks a chance to slow your target, but the ugprade to sheen is a big one since instead of 100% of your base attack power, it will give you a 150% boost instead. the lichbane will give you more ability power and magic resistance, but it will change sheen's effect to give you 100% of your ability power added to your physical attack. upgrading it to lichbane should ONLY happen when your AP will surpass your base physical damage, which will usually be the case since it gives you 80 AP on its own, but should also be more than 150% of your base attack power such that it'll be better than the triforce other wise, it'd probably be better to just get a triforce instead if your looking for damage
also pay attention to the tool tips of the items, there are a few keywords that can make a big difference, any item that say "UNIQUE passive" means that getting multiple copies of the same item (weither it be an upgraded version or the same copy, for example sheen and lichbane) will only allow you to get benifits from one of them. so its usually a good idea not to get multiple copies of the same item in this game. another keyword to look for is "UNIQUE aura", even though it says unique, this just means that you can not have the same item in your inventory and recieve bonuses of both of them, BUT if another team member has this item and you do as well, you will gain the buff twice. but only a maximum of 2. think of it like this, the item gives you a buff, while it emits an aura buff to everyone else, since the self buff is not an aura buff, you can have both. but an aura buff from multiple people will be the same thus you only get 1 copy of the aura buff.
in this game, all items stack, but depend on the stats, they may stack differently. majority of the abilities will stack additivly, while some stats will stack multiplicly (such as dodge). for movement slowing abilities, they do not stack, they will be applied to the target, but only the highest value will be active, once that debuff wares off, the next highest slowing effect that is on the target will take effect for it's remaining duration.
always keep a balance between killing power and survivability. getting items only increasing your attacks will leave you defensless to attacks yourself. this may be benificial in one on one battles where your able to disable your opponent and kill him before he can do much damage to you, but this is a team game and more than likly, you'll be doing team battles. in this situation, you can be easilty targeted and focused down on. so getting some sort of survivability item is key in team fights. this could mean getting an item to increase your health or getting more armor or magic resistance.