I've got 40+ Soraka games on TB so far, and I've tried most everything. Not exactly a Pro, but I win more than I lose with her.
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The choice here is if you need HPreg or gold. To start off with Soraka, I highly recommend taking Coin until you learn how to position. Once you learn to position, you should also be comfortable poking with her aa and q. Once you have that confidence, start taking Thief. The 10 true damage will really help you poke down people with your aa. Your range is decent, and easily abusive if you are persistent at it. I've made Braum's and Taric's cry with just my aa's. However, I don't recommend finishing either past the initial upgrade. Read below for why.
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Soraka needs mana regen and she needs AP. Both items suit her really well. However, you should be healing with your Q, not by standing around minion waves. You also do enough poke with abilities that Frost queen will earn you lots of gold all throughout the game. It's not uncommon for me, when I do make this item, to hit 1000 by 25 minutes. I like it's active because it helps Soraka land her Q and E, and is pretty much her only form of instant cc, even if it's only a slow. Q should follow right behind it, but Q isn't instant. Talisman is awesome team utility, but I find Soraka moves fast without it, and is more concerned with keeping her team healthy than diving into a fight. If you don't finish, sell the base item to make room for higher items.
However, I rarely finish either of these items. I prefer getting better utility items early rather than a support item. But if the lane phase goes on til 20 minutes, I'll grab one of these.
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This is your decision for a first item. Do you need to juice up your abilities and survive assassins, or survive extended fights? Warmogs is Soraka's only way to heal in between fights without returning to base. I find it very effective. Zhonya's is what will help you survive skirmishes and make your spells do more. It depends on the game, and how you see it playing out. Stacking some HP isn't a bad thing at all, it make enemy damage less intrusive to your already limited healing. Just don't build only HP. The main reason I build warmogs is for the 3% hp regen. You will really need it. Zhonya's is your main armor item. Even just building the core item (Armguard) will really help you. Having both warmog and zhonya isn't a bad build either.
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90% of the time, take Mobi boots. You need the speed to place wards in the mid game, and to get to the fights late game. Finish the core sightstone and upgrade it. You will be out on the map a lot, and vision is your game. Crucible is going to be your go-to mana item. Build it whenever you feel you need more mana.
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Here are your final item options. Since Soraka tends to get to low hp a lot, GA can really help you. BV helps keep you from cc, and Rabadons stacks on top of Zhonya's really nicely.
...................................................................Gameplay:
...................................................................Early game:
Q > W > E > W > W > R
Use your Q to zone out the enemy, and AA a lot if they get in range. Banana's are OP and you will do a lot of damage if you keep one of the other champions always low hp. I always focus the initiator of the two, which is usually the support. If you can single handedly zone the support, their adc will be unable to cs. Have your adc freeze lane, and enjoy some farm. I actually take AD marks with my spellthief to make my AA's hurt more. Rely on your Q to heal up up, but don't go oom with it: It's 1/5 of your mana pool early game.
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...................................................................Mid game:
Pick either Warmog or Zhonya's for your first item and start building it. Keep dragon and it's approaches warded. Your ADC should be able to keep farming with you gone for the occasional 15 seconds and awesome vision. Stick to your team's playmakers at this point. Anticipate the fights, and keep the dragon side of the map warded.
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...................................................................Late-Mid game:
This is about the 25 minute mark. If you find that you die early on in fights, get either BV or GA: Your life is more important than better heals.