Ok this is my first guide and a fairly long one, so please be considerate and take the time to try the build and supply constructive feedback. This guide is inspired by a game where I was playing against a GP and Akali who both could burst a squishy Soraka down very easily. Being a support character and if played half decently should be a target by the enemy team, so inturn I had to counter that building tanky mid to late game. With this build you have sufficient AP to make crucial heals, obtain full CDR and be able to take a hit, kite, heal and take more hits or more of a scratch.

Marks: I go flat ability power and NOT Magic Penetration. The reason is the ability power gives your heals that extra boost which is useful. Soraka is not there to dish out high amounts of damage so there is no need for magic penetration.

Seals: Mana regen/5 allows you to use silence on your enemies more often instead of focusing mana on yourself to maintain heals. Having that extra mana and being able to silence an enemy oponent when they move in for a creep kill makes them back off and take damage, this can lead to zoning the enemies from creeps.

Glyphs: Flat cooldowns. Obvious reason, more cooldown the more you can heal. Can be swapped for flat ap but I found that extra cooldown helped early game.

Quints: Flat AP, 15 bonus AP is very noticable early on when healing.

3-6-21
Ability power per level, more AP the more you heal.
Soraka is quite a squishy character, that extra magic resist and armour means you take less damage.
21 in Utility picking up the extra gold per 10 seconds, bonus movement speed, CDR and a bit of bonus mana regen to reduce the amount of mana you have to give yourself allowing you have that bit of extra zoning power.

Clair Voyance Clairvoyance is a must for Soraka. It allows you to spot enemy junglers, be aware of incoming ganks, or for that extra sight for a team member to scope a kill eg lux,cait,ez, noc, GP.
Flash Flash, if Soraka gets caught in the middle of a team fight she will get focused. Flash helps you escape from those sticky situations and also allows you to get closer to a team mate who is in desperate need of a heal.
Why I don't go for a Summoner Heal? Escape is needed with Soraka and I believe that the benefits of CV outweighs that of Summoner Heal when you already have 2 more than capable heals.

Note, Soraka's farm should be very little. Chances are very likely that you won't complete this build. In the many games of Soraka I have played I only ONCE obtained a full build.
I have tried games without a Doran's Ring Doran's Ring first up and Soraka is just too squishy with only 4 bars of HP. It also gives you that bit of extra mana regen allowing you to start your zoning and seemlessly infinite heals.
Next is Mejai's Soulstealer Mejai's Soulstealer. This should be all the AP you need. As a support player should know, you gain an assist ie 1 stack when you heal an ally before they kill an enemy champ. Soraka's world wide heal = easy stack.
Ionian Boots of Lucidity Ionian Boots of Lucidityfor cooldown, may be swapped for Merc treads if they have an early strong ap/cc team with a champ like fiddle.
Next you can either get Aegis of the Legion or Glacial Shroud. If the enemy team is AD heavy then go for Glacial shroud. If they are mixed or AP heavy then go for Aegis of the Legion. Glacial shroud with the bonus off CDR boots will give you an early max CDR. Aegis is necessary for a support as it gives your carries that bit of extra armour and magic resist which will go further than you may think.

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This is usually as far as my build gets before the enemy team surrender or their nexus is destroyed

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To get passed this point in the build you have to have numerous assists which will give you stacks and more than enough money to allow you to do your job well.
But continuing on, turn your Glacial shroud into a frozen heart which is brilliant as their AD carry does way less damage to you.
Finally a Guardian Angel giving you bonus armour and magic resist. And if the enemy team manages to track you down and kill you before your team kills them then you respawn on the spot, which then you will be able to heal yourself for plenty.

The final item slot is left open for wards. It is your job as support to help maintain vision over the majority of the map. This will help more than you can imagine.

Initially you should CV the enemy base. This allows you to see what items they obtain and sometimes which direction they are heading.
In a well structured team in my opinion there should be 1 Ad range carry, 1 Ap Range carry, 1 Offtank, 1 Tank and a Support. Where the offtank or tank usually jungle. The Offtank or Tank then solo top, Ap range carry solo mid as they need the extra experience to gain levels for that extra damage as they are spell reliant. That leaves the Ad carry to come bot, focus farm and harass without consequence. Your job at bottom lane is NOT to farm. Soraka should finish a game with > 20 farm. That gold at bottom needs to go to the Ad carry for their items. I cannot stress enough that a farmed Ad Carry is much more useful than a farmed support. So inlane you should supply mana and health to the Ad carry as they need it and at the same time silence the enemy champions as they come into get a last hit on a minion. Soon they will be at half health and you will be able to zone them and keep them off minions. You should stay in lane long enough to obtain Mejai's and if you are lucky boots as well.

Once a tower has been broken you start to enter the mid game phase. Here your job is to plant wards and obtain stacks and gold via assists. DON'T try to solo push lanes, farm up from jungle or anything like that, you are much better use when you stick with your team mates and get them a kill and yourself an assist.

Mid to late game you should have your Frozen Heart and Aegis of the Legion. You are now able to take a hit and live. The enemy team will be furious with your heals and want you dead and will most probably target you. This is where you become more useful. Kite, kite, kite and kite some more. The more they are chasing you around trying to kill you the more time your team has to put a hole through their carries leaving them with minimal output allowing for your team to push. If you get to the stage of obtaining a GA you are basically indestructable. You should have 13+ stacks making you heals generous, you will be taking minimal damage from their Ad carry, slightly more damage from their Ap carry and near to no damage from their tank. They will want you dead more than ever now but they cant kill you because you take low damage, any damage you take you can heal and they don't get the chance to deal enough damage as by now your team has killed them all.

Pros:
You can heal your team mates generously.
End game you can kite amazingly.
End game you are near impossible to kill.
Heal some more.
Cons:
Don't expect to carry a team to victory, your damage dealers have to be good or will lose the match.
Early game you are slow and squishy, stay back if you think there is a chance you may die.
You will be a target although late game this will be good as you are hard to kill.

You're job is to get the carry farmed early game, early/mid game onwards to ward the place up, mid/late game to kite the enemy into thinking they can kill you allowing your team to get easy kills, late game to heal for 400+ regularly and soak up a little damage. Don't take any farm. EVERYONE else NEEDS it more than you do. Most of all have fun when you kite the enemy team when they think they can kill you, get those heals in at the last moment keeping your team mate an inch from death allowing them to get kills and most of all have fun in silencing the enemy team and watch them walk at you not being able to cast any spells :)

Hope this guide helps in your gameplay and enjoyment when playing Soraka!