Welcome to my build guide for Soraka! Now she LOOKS pretty easy to play. Run around, land a few starcalls, heal a few champions, stay in the back. What most people don't know is that Soraka has one weakness, and this weakness is called getting hit. If Soraka is attacked, that means less heals, and god forbid it's any sort of CC, because if it is, you will hate life.

Runes.
Some people like putting a mixture of things in their rune page.
I am not one of those people.
My runepage is set up so that you can lean on it in the late game, when you'll actually need that extra 85 AP that it gives you, frees up slots you would usually be using to increase your healing power.
That is why I have a full runepage of scaling AP.

Every mastery here is selected specifically to increase healing ability. Wanderer gets you where you need to go faster. Meditation , Recovery , and Perseverance increase the speed at which you gain your healing resources. Runic Armor increases the healing from Starcall, which means more heals you can dish out before throwing down another Starcall. Tough Skin and Secret Stash increase the potency of damage mitigation techniques early on, allowing for more heals. Bandit gets you gold, which you'll need as a support, and Windspeaker's Blessing gives your heals more power.

Each of these items in the core build shares one very important, very irreplacable trait: they make you run faster. The three AP items give you nice little extra effects if you actually end up doing some damage in a teamfight, but for the most part, their most important quality is the move speed bonus they give you.
Boots of Swiftness - Alacrity - Increases your speed by 85. I don't usually build boots, but on this build, it's imperative that you build these specific boots.
Lich Bane - 7% move speed, 80 AP, 10% CDR, and 250 mana. All these stats help Soraka's healing ability in some way. Yes, the passive is probably wasted most the time, but we are more worried about getting across the map in 10 seconds to that fight that broke out in the top lane so that we can heal everything.
Luden's Tempest - 10% move speed, 100 AP, and a nice little passive that helps your ADC melt the enemy when you land a Q or E. More useful than Lich Bane when it comes to usage of each part of the item, but we are still most concerned with the move speed.
Ardent Censer - 8% move speed, 40 AP, 10% CDR, 100% mana regen, and a passive that turns your ADC into an unstoppable nightmare. There isn't a person in League that'll get upset with you for tacking on 15% attack speed and 30 magic damage on hit to their basic attacks on top of the armor and mr that Windspeaker's Blessing already sticks on to them.

As far as gold generation items, either one will do you good, though I would suggest the Eye of the Oasis because it gives you wards as well as health regen and more reliable gold, and even though Talisman of Ascension increases move speed in its active, everyone knows that if you don't have wards, the entire team will hate you.

The last item is really up to you, though I would suggest a heavy AP item like Rabadon's Deathcap to keep up with the massive damage that is bound to be flying everywhere if you have gotten this far into the match. You can TRY to help with waveclear using Nashor's Tooth, but you're better off just using Starcall and procing Lich Bane to keep waves in line. Besides, the last 10% CDR is not really all that important. you have 35% CDR with this build. You could build Will of the Ancients to get a little more heal in and some more CDR, but really it's just more effective to have massive AP or more health by now.

If you find the match dragging on, and both teams at a stalemate, then perhaps it's time to sell the support item in favor of Mejai's Soulstealer. The extra AP and move speed gained from this item will increase your healing and kiting ability to godlike status, not to mention make your abilities REALLY hurt anyone who thinks you're just another squishy target.