Not Updated For Current Season
Basically, you start off with the sapphire crystal and as soon as you get 925g you port back and get your catalyst. That should secure you in your lane until you finish your Rod of Ages, healing and giving you mana every level from around 3 or 4 to about 10 or 11. By that time you'll probably want to start pushing with your team (once you get boots).
Stay in the back, finish the item builds, and make sure you get golem every time it's up. You with golem means you can sustain your team for two consecutive pushes, giving them mana, and hp at every cooldown.
Heal order should be:
squishies at 1/2 life > carries at 1/2 life > tanks at 1/3 life. Keeping your casters/carries above half life gives them security to damage, whereas tanks should be able to take 15-30% more damage and have more life in ratio. 1/3 of your tank's hp (1000) should over half of 1/2 of your dps's hp(800).
Stay alive, don't stick your neck out, and save silence for anyone that comes running into your team.
Lastly, the heal spell is just one more thing to keep your team in its lane. You should be able to push a bit further just because of that heal. The flash is super important. Flash will keep you out of harm's way if they get the jump on you. If you die, your team suffers more than if 1 of the other team's members die...because you're Soraka.