The main advantages of Soraka over other supports are that she can give your ADC mana back (giving them super-long sustain in lane), and she can heal teammates all the way across the map with her Ult.
Q: Rains down damage and temporary reduced MRes on all nearby enemies. Playing Raka in a support role, this will be your least important skill, likely picking it up at level 8 and not leveling it further until required to.
W: Heals a target ally and gives them increased armor and MRes for a few seconds. Unless your ADC is horrible with mana management, this should be your main skill to level whenever you can.
E: Can be used to either restore mana to a teammate, or deal minor dmage and silence for a bit to an enemy. In laning phase, you'll generally use this for the mana except when going for the kill. Since it cost no mana to use, feel free to use it to top off your ADC whenever they are below about 75% mana. This should be leveling priority 3 (behind R and W)
R: Instant healing to yourself and all teammates, wherever they are. Typically a last resort heal when you are present, but you can make some stunning turnarounds in the lanes with this if you watch your teammates health bars constantly. When you get into teamfights, don't be too conservative with this; use it to either keep the initiator alive, or whenever 3 people on your team could use healing.
Runes and masteries are pretty standard support stuff with Soraka. Runes should be either full tank (Armor quints, yellows, and reds, with Scaling MRes blues) or money (Gold Quints and yellows with Armor reds and Scaling MRes blues). Masteries at 0/9/21.
As far as items, again like pretty much every support, your first priorities should be PhiloStone, basic Boots, and SightStone, in that order, while buying up to a few potions and 3+ wards on each back until this is done. Then upgrade to Lucidity boots. If you are having mana issues, build a Chalice here, then build a Locket. If your Top or Jungle aren't building toward an Aegis, do that now. Then build Sherulya's.