Not Updated For Current Season
First something importorant:
Cooldown reduction for heals and infuses ervery 4 seconds, your teammates will never run out of mana or hp.
Push Carefully, and watchout for stealthers, those are your worst enemies.
Always keep either a tower or a friendly player in range of you incase a stealther runs up and attacks you.
By this time, you should've got all the item up to the locket. So you survivability should be abit better now, but not perfect. You could buy a Oracle Elixir now if they got either Twitch, Evelynn och Shaco on their team.
Keep an eye on your teammates Health so you know when to use Wish.
Use it when 2-3 of your teammates got low Health to help them push instead of going back to the Camp. Or use Wish if one of your teammates is about to kill someone and he got very low Health.
By now, you should've gotten all the items. Help your teammates push even harder now, if you grouped up with a good AoE champion at the beggining, you should've entered their camp by now.
If not, help your teammates push, doesn't matter which lane their on, just help.
The most common mistakes I've seen other Soraka players do, is to use Infuse on their self when they got ~70% mana left. Save Infuse! Either for yourself, or for your teammate/s. Using it on your teammates will make you able to push even harder on your lane. Using it offensivly is also a great idea.
Another common mistake is to just waste Wish on yourself, instead of saving it and help your team. This is unnecessary.
Another mistake, not that common anymore, is that players run out of mana by spamming Starcall on mobs, and then get attacked. Just use Starcall on mobs now and then, not all the time.