AP Soraka is a very versatile champion that excels in the mid lane. She is a safe pick that holds her own against almost any champ and can easily snowball into a late-game Support-Carry hybrid.

Pros:

High early damage potential and harrass
Can force her opponent out of lane/out of experience range
Passive Magic Resist
Heals/Mana to extend your teams life in Teamfights

Cons:

Limited mobility
Not a true carry
Vulnerable to stuns, heal reduction
Hard to convince team you're legit

The Early Early Game

At level 1, 2 and 3, Soraka has a huge advantage. This is the time where you should be most aggressive. Each time you hit your opponent with Starcall you reduce their MR by 8. At level 1 this is huge. Your opponent will have one ability and a much longer cooldown. You would be surprised how many times you can get first blood with a couple of Starcalls and a Flash Ignite Starcall for the kill. Level 2 and three are the same, except now you can silence them and heal any damgage you take, letting you win any exchange.

Zoning
If you get them low early, and they don't want to leave lane, you can zone them out of farm and experience. Get up in the enemy creep wave, and Starcall anytime they get close. Be careful not to take too much damage from minions, and not to be out of position in case of a gank. It's best to have your lane well warded when you are zoning.

Wraiths
Don't starve your jungle, but if you see that they are top or bot, and your creeps are at the enemy turret, go kill the wraith camp. It's easy for your lane to get pushed as AP Soraka, so go get wraiths while you wait for them to get back to center.

Deceptively Unsquishy
Opposing teams will think that you are easy to kill, but with consecration, the bonus armor from Astral Blessing and Wish at your disposal, you can bait enemies into diving you. Be careful if they Ignite you though.

Some folks that you counter pretty hard
Zed - Use Infuse to silence him immediately after he uses Death Mark to mitigate most of the damage. You can also use Astral Blessing and/or Wish before the delayed damage hits.
Vladimir Your heals match his sustain. Infuse limits his use of his abilities. Use Wish before Hemoplague damage hits.
Karthus Soraka is basically anti-Karthus. It's easy enough to dodge Lay Waste, and harrass with your own abilities, and all you have to do is use Wish whenever he uses Requiem. (P.S. Wish has a lower CD

You will also do well early on against champions that need to be in melee range to auto-attack, such as Gragas, Fizz, and Diana but be careful of their burst post level 6.

Synergy

Ideally, when you play AP Soraka, you team will have another AP champion. The MR reduction from Starcall let's your AP teammate go in and do massive amounts of damage. I particularly like having a Singed run around with his Poison Trail on, while I am spamming Starcall.

Your Role

It's important to understand that AP Soraka is not a true AP Carry. Even in games where you get fed, you might end up with a score like 7/2/20. This is perfectly fine. The build style I have provided makes you an AP bruiser; you have quite a bit of tankiness as well as medium damage potential. Add the utility of the heals in team fights, and you are carrying the game in your own way (e.g. Wish can heal each of your teammates for ~750 health late game)

Starting Items
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Doran's Ring+ 2 Health Potion should always be your standard start. You will be able to stay in lane for ages with the extra health and mana regen this item offers.

The only time you might change this is if you face an AD champion such as Zed, Pantheon, or Talon. Starting Cloth Armor and Health Potion 5 is another possibility.

AP Soraka should adapt her build every game based on the opposition. Take a look at your opponent and their jungler. If you're facing and AP mid and an AP jungler, rush Chalice of Harmony or even Negatron Cloak if you have a lot of jungle focus. If you are facing AD, rush Seeker's Armguard. Due to Soraka's lack of mobility, and tendancy to push her lane, you should expect a lot of ganks. Taking the jungler into account when adapting your build is essential.

Essential Items
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This item is important for it mana regen and CDR stats. Choose it over Morellonomicon because of the MR.

Once your Starcall is level 5, you will begin to run out of mana quickly. Getting this item mid game so the passive can stack is important.

Given Soraka's lack of mobility the slow from the passive on this item gives her more escape potential, and also can slow down fleeing enemy's to secure kills.

Other Items

This item is not always essential, but a smart team will try and focus you so you can't heal your teammates. Using the passive of Zhonya's at the right time, takes the focus off of you, and at the end of the two seconds, you come back in with Wish. The Armor stats are also important because Soraka's base armor is very low. Make sure you keep Seeker's Armguard long enough to fill the stacks of Arm and AP.

This is a late game item that will boost your damage if you already have Rylai's Crystal Scepter because of the augmented passive on Liandry's if the target is slowed. You don't want this item if you don't have Rylai's.

Situational

If they have a lot of AP champions, or if your lane opponent is bullying you.

This item isn't ideal, because the passive is scales off of base AD, but if you need extra armor then it can work, and the passive still gives you a slow on your auto-attacks. Alternatively, you could go for Frozen Heart. Even Guardian Angel comes in handy as a secondary defensive item.

Get this instead of Rod of Ages if you feel like you don't need the extra health.

This is a fun item to get, and will help you practice paying attention to your top and bottom lane. You can fill the stacks on it by getting global assists from Wish. Make sure you stay alive though!

Only get Rabadon's if you get very fed. DFG can replace Athene's in the very late game if you have gold to spend after your build is done.

Boots

I would recommend these boots 90% of the time because of Soraka's limited mobility. If you're dealing with a Fiddlesticks or a Rammus then you might want Mercury's Treads for the tenactiy to reduce the time of the fear/taunt.

Max: R Wish, E Infuse, Q Starcall, W Astral Blessing

It is definitely better to max E before Q. One of the best features of AP Soraka mid, is her incredible sustain and ability to stay in lane. Level 1 of Starcall costs 20 mana and has a CD of 2.5 seconds. Soraka's base mana regen is 6.8 per 5 seconds. Add 3 to that from Doran's Ring as well as the 4 mana returned to you for every minion kill.

Now 9.8 per 5sec is the same as 4.9 per 2.5sec. If you're killing 2 minions per starcall, that means you get 12.9 mana back out of the 20 used by the time Starcall is off cooldown. As long as you don't use it every time it's up (which you shouldn't), you will never be starved for mana. You can even let your jungler take the second blue!

Infuse has a longer cooldown, but the damage on it scales better than Starcall and best of all, it costs nothing!

I usually max Astral Blessing last because it's most effective in late game teamfights, and early game, I often have Health Potion in my inventory. You might want to take a second point in it before fully maxing Starcall though.

Wish You will need to become adept at keeping an eye on your teammates' health bars, to know when to save them, and also to be able to pan your camera to top or bottom to see when you can snag an assist with your ult. Make sure not to use it too early, and if the attackers happen to be full health, you won't get the assist.

Astral Blessing Keep in mind the 3 second armor buff as well as the heal. It has a long cooldown, so in general, use it when it's up if someone needs it, but if you're about to battle, save it until your teammate is about to go into the fight.

Infuse As AP Soraka, you will use this almost exclusively on enemies, but keep an eye on your teammates mana bars just before a fight, or when you're at dragon or baron nashor . If you're just a little behind a fight, using Infuse on an attacking teammate will also net you an assist.

Starcall Get to know the range of this ability. In lane, there will be times, when you don't want to hit the whole creepwave, and times that you do, so practice positioning. Same goes for teamfights - it's rare that you'll be able to be in a spot that lets you Starcall the whole enemy team, but it might be worth it to flash in once in a while if you have your Zhonya's Hourglass passive up. Don't forget that the MR reducing passive on Starcall stacks up to 10 times.

consecration Your passive gives you and your nearby teammates 16 MR. Make sure you are nearby during teamfights.

Soraka is a safe pick with very high sustain in lane. She is easy to pick up, but make sure that you get used to the range of Starcall. Also, her bananas move very slowly, so you need to get used to when to fire in order to last-hit - this is essential, because if you just use Starcall your lane will always be shoved, and the enemy jungler will Feast on you.

I hope you have found my guide useful! Please let me know of anything you disagree with or changes that should be made.