as well all know, soraka isnt a killer so this is an assisting build where you help the team as much as possible without dying since hes the most targeted character in the game.
not only do the other team target you cause your a healer, but they tend to underestimate a soraka and think they are food =]

with my build:
pros
-low cooldown heal spams
-decent healing(global)
-able to allow allies to tower dive
-not squishy

cons
-targeted by everyone
- everyone hates you
- no potential to kill

i dont really care much about my masteries as they are just for extra survivabillity and cooldown reduction. you can adjust your masteries whatever way you want

i decided to go 2/21/7 for my masteries. the last 9 to me is unimportant and just extra regen and ap is beneficial. for me, the 4% damage reduction is the most important thing as soraka is most targeted hero in your team.

i went teleport and heal for my summoner spells.
teleport-fast way to get to your allies,since your going to be targeted early game, its a good way to get back without loosing the important early exp
heal- for me an extra heal is just too awesome, i mainly use it to save someone or just to get team full hp for a team fight

other choices
ignite - meh, most likely if you ever going to get a kill, your allie would just ks, which is preferred since your not meant to get kills
ghost - good escape mechanism your choice if you like it
flash - same with ghost, id prefer flash over ghost due to the instant distance covered
clense - usefull to get away from ganks
exhaust - help allie get the kill ^^

with everyother hero, try last hitting the creeps, if you are an able to manage this, use infuse to kill the creap, just keep in mind it has a 15 second cooldown.

*** my fave technique***
to kill the stupid towers, HEAL THE TANKY CREEP - wait till its low hp and then heal it since it heals it full hp + gives it an awesome +80 def heheh which will then make it last about 10 htis = you and allie own that tower XD

try and use infuse to threaten as much as possible or use it to heal your allies mana so he can constantly threaten = more creeps to farrm

since soraka will be lacking exp due to low creap kills, use your ulti at lvl 6 when your allie is going to kill/ die = this will save him or give you an assist which will ultimatly result in exp and money muahahahah
main source of money is from assists

YOUR A HEALER so dont be afraid to heal =.=
your allies are going to be so much more important than you,
since you have 3 types of heals:
use blessing on the dps or carry = your carry now has extra 80 def = he can survicve SO MUCH MORE
use your summoner spell to keep tank alive since he just needs the upper edge
use ulti if your idiot allie starts running from your casting area.
since im using ss in this build, healing everyone will give you so much stacks

silence the casters
try silencing chanelling heros such as nunu with his ulti, fiddle, veiger and healers