This is a repost of a reponse I mad ein another thread but I figure its valuable enough to up here.
First off, for this hero, you need a lot of practice with skill shots and micro managing, this hero requires a lot of clicking to maximize her.
I start with boots (cause she so slow, speed helps with dodging enemy skill shots, and positioning yourself to land skill shots) and pots ( 1 health 2, mana pots) then get sheen. With these 2 items you now have enough damage to srsly wreck any hero in the early game (you might want to grab blue if you can, ask jungler first.) (or carry an extra mana pot early game, it may get you a kill, or save you or an allies life).
Max q, followed by W, and finally 1 point in e for the cc, ideally u want as much burst as possible early game, hence q and w maxing, and get ult every time it comes up. Ok, now here is why sheen is mandatory, because after each skill you use, you need to land that auto attack, and make sure you have 2 seconds apart on each skill usage. 1 exception When they are half health, I go in for the kill, I lead with Q and AA (auto attack), then pop W and ult near to them, AA, ult again, AA, Q is up normally by now, finish them with ir, and save last ult to get away, or use it to finish them. This will kill most heroes if executed properly. ( I waste an AA between W and ult because they start running as soon as you ult towards them, so I rather have it land in a huge burst as a surprise, losing large chunks of health and being charged encourages a run response in most gamers.)
With those 2 items you should be able to get some easy kills, if you are good with Skill shots, micro, and move attacking. If you are not good at these things, you can either practice, give up playing her, or play her and fail.
Depending on how you are doing, you can go to teir 2 boots (Sorc is what I usually use, but CDR boots not bad either) OR you can go upgrade it sometime later, your call.
Next item - her damage is perfect where it is, you need some beef, get Giant belt, and then turn it into rylai, this is amazing cause all your skills now slow = Skill Shot easier to land, and she has survivability, and some AP, im not a fan of massive AP on her, she has really bad scaling, this is why after Rylai, I personally build Lich bane right away, because 150 - 200 extra damage on every auto attack between my skills is better than 70 or 80, and spending tonnes of gold on AP is not going to be as effective.
After those items, you don't need much else, you can go Spell vamp but I hear it doesnt work well with Q on return (true damage can't proc spell vamp) and also does not stack with passive (rumour, not confirmed) You could go zhonya's if people are targeting you, basically, anything you want, now your AP scaling is gonna be great on auto attacks cause of Lich bane, so AP becomes much More viable.
Some people say Cap first, I disagree for reasons Above, good luck pwning.
Oh btw I run flash and ignite typically, also I run mpen reds, armor or health yellow (health more effective for mid, or in general cause it helps vs AD and AP, again imo, and I don't want to buy more runes right now lol, I also run mp5 scaling blues cause its best I got for caster runes, and I run Mpen and Health quints, as you know, I don't favor AP on her, although if you want to sacrifice early game, you can pump AP runes for when you get lich bane, ie what Phreak did in his Preview. You can't go wrong with moar healths.
many people may disagree with the following. I run 9 offense 21 utility, why? Cause I don't care much for AP on her, (is there and echo in hurrr?) and I rather have 10% Cooldown, that tiny bit of speed, more mana, mana regen, and faster leveling is actually OP, Why? Because if you only lane equally to your opponent you will get your ULT first, by a significant amount, enter snowball effect here.
Whats 6 % damage on a almost dead champ? hmm on 300 damage its 18 damage, wow so useful, OR.....oh look I got my ult first, PWNED!