Soraka is Squishy, you can go full tank but I guess you'll still die in 3 hits. STAY BEHIND YOUR TANKS/FIGHTERS!
If you feel comfortable enough go in and Starcall the enemy Team till they... well Die :D
You will be shooting these Starcalls like an AK47 - Watch out for your mana though so you still have enough to throw heals around!
After a while, when your enemys are not completely mentally behind, they WILL focus you. Why? Because YOU ARE ANNOYING - and that is GOOD! Use it to Bait them into Fights, if you die and get atleast a kill for your ADC it was WORTH IT!

Learn who and when to heal! - This is EXTREMELY Important, even more important than staying behind.

Time your Silence on their biggest Threat (for example, a fed Vi is charging her Q - Silence her and stop her from Innitiating ;))

Your Ultimate will be the most OP spell you got, since when you have your Ardent Censer your team gets a WHOPPING 25% ATK Speed increase for SIX seconds . again, learn when to Use it for maximum effectivity!

Onto the Items:
First and Foremost - You can only get 40% Cooldown Reduction so calculate beforehand which Items with CDR on them you will get (10% from Ardent Censer and 10% from Masteries!) and do not necesseraly get more than 40% except for when you are really fed. Well, you could always get a Zhonya's or Mikael's though!

Faerie Charm is the Base to most of your Items, it gives nice mana regen (which you really need) so if you can get some early and build into finished Items when you got the Gold.

If they have many CC Abilities and focus them on you - Zhonya's Hourglass
If the said CC Abilities are cast on your Team go for Mikael's Crucible. And PLEASE remember to actually USE IT!!!
Frozen Heart if the Tank on your Team doesn't have it and you get kicked by their AD Champs often. Banshee's Veil if you get Fireballed by their AP Champs.
Spirit Visage for MR and giggles, like Trinity Force and Lich Bane... Only get those last 3 if you're playing with friends ;)
Boots are pretty self explanatory: Get whatever you NEED in the particular Game!

Thanks for Reading!