At the beginig buy Boots of Speed, and choose silence spell. Many people take heal firstly but it costs a lot of Mana. By moving fastly you will be able to silence enemies, furthermore deal them damage without taking huge damage on yourselves. You need to harrash enemies and try to kill them. Even if you dont kill them, they need to go back , so they loose their experience.
Why i take Ghost and Flash? Soraka is really squishy champion and it can make her running away, or you can do turret diving, kill enemy with low health and run away with flash.
When you are taking damage, just use potions. I make Soraka as Tank - Support. I am not diving into full AP. As I said, Soraka is very killable champion, for sure Mercury Treads will solve some problems, espesially when oppent team have much cc. If not, buy Ionian's boots. I focused to make the least cd reduction as i can, this allows me to heal (in the medium and late game) and silence much more and Soraka needs some health, so I suggest to buy Kindlegem. Furthermore we need HP regen and MP regen, thats why i made Shurelya's Reverie and Spirit Visage.

As time comes to push the mid lane, and do some teamfights then comes medium game. Here you need to make yourself more tanky and better support. Thats why You need some AP. I buy Morelo's Evil Tome, because it gives 75 AP, 12 MP regen per 5 secs and gives 20 cd reduction.
It makes your heal and silence's damage stronger. Overall MP points can be too small so to make it more tanky, and have more MP I make Frozen Heart, it also gives cd reduction and is very nessaary against dps. I think the most times you will be focused so try to not participate the main battle, and act from distance, supporting your team to win. As the next item buy Rod of Ages which will increase HP , MP and AP.

As soon as you are pushing inhibitors, there is time to make Soraka more and more tanky,
to make yourself surviveable and hard to kill make Guardian's Angel. I think this just make sure to survive, because on this stage, its hard to kill such Soraka. Usually I sell Spirit Visage to make slot for GA, but if its not needed, I stay at the ROA last item.