I strongly recommend starting with Soraka because:
- Cheap (450 ip)!
- She is the best healer in the game (can restore hp and mana to others)
- Her ult promotes map awareness
- Not CS/item dependent
Most people want to start with carries, like Ashe or Annie. I think it's much better to start with support. Playing support forces you to be aware of the game around you, instead of focusing on doing damage.
Recommended items (for a beginner): WARDS, Philosopher stone, Mercury Treads, Heart of Gold, any upgrades for these items, anything else with AP, Health, or Auras. Try starting the game with a Faerie Charms and wards. Ward the river, and focus on harassing the enemy w/out taking damage and healing when you can.
For runes, get armor seals and magic resist glyphs. For masteries, try 0-21-9 or 0-9-21. Make sure you get the extra health in the defense tree, mana regen in utility, and cooldown reduction anywhere you can.
Soraka's job in the game is to support a carry. You'll want to team up bottom lane with a ranged AD: ashe, caitlyn, etc. Your focus is to ward bushes, heal your partner, and be a nuisance to the enemy team. You want to get items that generate gold to compensate for not getting very much CS.
Finally, keep an eye on the minimap. If you see an ally fighting someone, try to steal a peak at their HP. If they're close to death, use your ultimate abilty (the one you get at lvl 6) to try and save their life.