IMPORTANT! This guide is NOT complete and is UNDER CONSTRUCTION!

Hello mobafire visitor, welcome to my first guide. I will update and hone this guide because it surely has its mistakes since I'm not level30 yet and I will gain more experience with playing this great champ.

This guide is dedicated to novice and advanced support players and is meant to be played in solo or pre-made 5v5 battles.

So, you've apparently decided to try the support role and to help your team with some heals and mana replenishment. Good job! Soraka is a pure support char and you won't get much kills if any. However, it doesn't mean you won't win, au contraire. With this build Soraka can't do much damage and is not able to carry or to solo line. You need a lane partner who you will provide healt and mana replenishment + silencing enemies and you need to secure good conditions for partner's farming so he has enough money to buy good items (It doesn't mean you are not allowed to farm if your partner is not here (backed to buy items or ganking another line/or dead which is probably your fault).

Ok, so you can use AP to boost your heals. That's the reason to pick Greater Glyph of Scaling Ability Power, Greater Mark of Scaling Ability Power and Greater Quintessence of Scaling Ability Power which aren't flat but you gain more AP on lvl18. As Xenasis commented, if you use flat runes you'll have more AP in majority of the game so you can choose Greater Mark of Ability Power, Greater Quintessence of Ability Power and Greater Glyph of Ability Power instead.

I found myself out of mana during early game quite often waiting for Infuse to cool down so Greater Seal of Replenishment is a good choice. If you want more mana regen in the late game the Greater Seal of Scaling Mana Regeneration is the right choice for you.

As you can see i go 9/0/21 to grab 9% cooldown reduction.

I pick Meditation , Perservence and Expanded Mind because of mana problems early game.
Awareness is a good mastery especially if you're roaming and every experience point from creeps is good. Also the more levelled Astral Blessing is the more you cal heal :)

Foreword: I prefer to build AP items to Armor/Resist items because your spells have great AP benefits (except Starcall). If you are smart and don't push lane alone, you will manage to survive even without armor and resist.

Early Game

I start with Doran's Ring as it gives you a little AP, Mana regen and HP.

I usually don't get to base until around level 10-12, but if you do or if you die, build items for Mobility Boots, Fiendish Codex and Kindlegem.

Which boots you shall buy:

Any comments here appreciated
Mobility Boots are good for roaming and fast lane switching (if any lane needs heal or help). Use it with wise combination of Clairvoyance to prevent running into enemy jungler or ganks.
Ionian Boots of Lucidity give you solid 15% cooldown reduction and are a good choice if you don't roam, but you can have your cooldown's reduced only up to 40% so it collides with other items.
Boots of Swiftness is another good choice if you don't roam. Gives you Enhanced Movement 3.

Mid-game

Fiendish Codex gives you AP + mana regen + 10% cooldown reduction. If you need money for other items later on, you can skip Morello's Evil Tome which is only upgraded Fiendish Codex.

Philosopher's Stone should be your other item as it gives you health and mana regeneration and generates 5 gold per 10 sec.
Blasting Wand gives more power to your heals through AP.

Now you can buy Morello's Evil Tome to gain more AP and cooldown reduction or skip it, but be sure to have enough room for the other items.
You can also buy Shurelya's Battlesong and use it's Active in teamfights or as escape mechanism.

If you're being damaged hard during fights get Cloth Armor, Null-Magic Mantle or Chain Vest so you can turn it into Guardian Angel later.
Guardian Angel is a great item if you are being focused in teamfights. Save your Wish and use it once you're revived (if others survived).

Late game

Now that you've built a solid items to have enough cooldown reduction and sustainability, it's time to boost your heals.

Build up Rabadon's Deathcap with Blasting Wand and Nerdlessly Large Rod. (90% of games ends before you finish Rabadon).

If game got this far or you got fed well, you can sell Doran's Ring and exchange it for another item such is Rylai's Crystal Scepter to combine with Infuse or Starcall. You can also grab Banshee's Veil if enemy team has a powerful caster.

P.S.:

Feel free to buy Mejai's Soulstealer if you are able to stay alive. But 20 stacks are a long way to go if you don't steal kills with Infuse. Then you can skip Morello's Evil Tome but Fiendish Codex is recommended.

As a good support you may want to place Sight Wards so if you have enough gold left after buying some of the items obtain few of these.

At first you need to max your Astral Blessing and Infuse spells to support your laning partner properly. I used to start with Infuse as first ability but then I found out that it's more useful to start with Astral Blessing if your partner gets damaged in first harassment.

I prefer to have my Astral Blessing one level higher than Infuse.

I leave Starcall to be levelled as late as possible because i find it useless as you don't farm. Also it's a pretty mana eater and does minimum damage. Speaking about reducing mana resist - you don't want to get close to your enemy to cast Starcall because you're squishy and one stun/slow/knocked air can get you killed in no time.
However, you may spend one point in Starcall earlier if you have a mage in your team and he's on your lane or you are persent with him in fights.

Since you are lvl6, always keep an eye on your allies' health bars (on allies list in the left corner of your screen) so you don't have to move with your camera to see who has low health and needs wish.

In mid game your Astral Blessing is your bread and butter spell as ganks are more often and push is needed. You can greatly improve your team's sustainability so you can push one more tower before you need to port to base.
Also, if a teamfight is on, a well timed Wish can determine the result of the battle. (Especially if you see Karthus channeling his Requiem)

If you've done good work so far you may find your team winning after opponents' surrender.

In late game your Starcall may become useful as you for example clear abandoned lane from creeps. If you time it well you can last-hit a lots of minions and raise money for another item.

P.S.: I prefer to save Infuse for silencing enemy casters (such Vladimir or Brand) in the right moment to prevent them from harass or combos.

Recommended

Fortify: You're a support and fortify can be very useful during risky turret dives or while enemy is pushing with a few creeps. Remember - no-one can win without destroyed towers. It can be 20-40 on kills but if enemies don't manage to push towers they are screwed.

Clairvoyance: Very good spell since it has a short cooldown. You can prevent ganks if your teammates report misses and look if the other team isn't at Dragon or Baron. Also it greatly improves your map awareness. Don't hesitate and spam it all the game. With the right use of Sight Wards you will always now what's happening on the map.

Heal: Why another heal? For your own purposes or if your team needs a massive heal. It only restores 50% of the health amount to your teammates so I use it through early game when my Astral Blessing isn't so strong.

Clarity: If you know you're going to be on lane with carry which uses mana and is always in shortage of it (such Ashe) you can use this one.

Flash: On higher levels you may get focused and Fortify won't help if opponets stick together and push. Also flashing through walls may be a good way while leaving or entering a battle.

Not recommended spells

Ghost: you don't really need it.

Ignite and Exhaust only for AD Soraka build (which this build is not).

Smite are for junglers and Soraka can't jungle.

Rally: i don't see any reason why to pick this one.

Cleanse won't help you. Ganked Soraka = dead Soraka.

Revive along with Teleport may be a good combo but blocks you from better summoner spells.

As I said before, you can greatly improve sustainability of your team if you stick together in mid and late game. Focus on squishy chars first if they're getting damaged. If not you can heal your tank.

Basic tactics which i use in our pre-made 5v5 team is that we gather to push mid and we go tower after tower. Tank iniciate, you heal him once if needed and if we succeeded we fall back and I heal the whole team so we are ready for another push.
If we haven't succeeded I wait for the right moment to cast Wish and to silence opponents with Infuse. Then I try to heal as much as possible until the battle ends.

Also your great ability is to kick champs from channelig/casting their ultimate. I love to watch how Nunu & Willump, Karthus or Fiddlesticks do no harm after getting silenced by Infuse. I recommend to save Infuse if these situations are likely to happen.

Thanks for reading and for constructive criticism.

I will update this guide as much as possible with my new experience, tactics and item alternatives.

To sum up:
Early game: find a suitable lane partner and let him farm. Upgrade your Astral Blessing and Infuse. You may roam to help lanes if needed.
Mid game: As ganks start to happen be sure your Wish is ready to help your ganked mates survive even if they're on the other side of map.
Late game: Try to heal as much as possible in team fights. Use your Shurelya's Battlesong too.

Special thanks to:

Xenasis
Halipupu