Taking Mid lane an AP carry Soraka has the Ability to carry and Pop out an AMAZING Team heal with the Amount of AP you get. Starcall can easily change the tide of battle and your other AP dps will love you for all that magic resistance shredding on the enemy.

for the runes i use a mix of AP/level and flat AP.
you are free to change it up if you want but i found this works well for me.
this set up with the masteries has me starting out with 10 AP no items.

this gives me a good enough amount of late game AP to keep people decently healed and enough damage with the starcall and Infuse to help with kills.

i run with a 9-0-21 set up grabbing the Magic Pen, grabbing mana/mana regen gold per/10 and extra starting gold. allows you to start with the Dorans ring and Mana pot. giving you more AP/mana regen and health

Starting off with Dorans ring and an Mana pot. you should be able to lane effectively until about 1.5-2k gold. then you go back and start working on Will of the ancients and your Mercury treads. after finishing WotA and your boots, i like to start Banshee's veil for the mana, health, and M.R. building Rabbadon's Deathcap last.

I start off with Starcall gabbing infuse at two then astral blessing at 3. Maxing out Starcall first and then Infuse maxing out astral blessing last.

Maxing starcall maximizes your damage output and is great when paired with Will of the ancients. helps to massively increases your lane sustain.

I like to carry Ghost to help in those tight situations when fighting is not an option. then the other is completely the Summoner's Preference, but I carry either Ghost or Exhaust.

Increased Damage and the complete ability to make opponents just go... "wtf".. especially when you draw first blood in Mid. you may be targeted first due to the great support and damage you put out but that allows your AD DPS and other AP DPS to really shine.

you are shredding enemies health and MR constantly in team fights throwing out amazing heals to members in need. and an super team heal ultimate. Allowing your AP dps to do more dmg and AD dps to attack undisturbed while the enemies to focus you.