Soraka can be a really fun champion to play, despite her being a full support. You just have to build her right. With this build you will have lots of movement speed, but enough ap to heal. Gona say it right now. This isn't a troll build -_- just because you get movement speed doesnt mean its a troll.
Pros:
Nuke heal
give mp
global heal

Cons:
slow(fixed with this build)
squishy
long cooldowns

As for runes get Cooldown Reduction and Ability Power. Soraka's heals have a long cd, so you will really benefit from the cd runes.

Not much to say. Just get 9/0/21 If you want to get exhaust, switch from crit to exhaust on offense tree.

soraka is really slow so you want boots fast. If you have no runes, buy the ionia boots(the ones that give you cd reduction). If you have runes go for boots of swiftness. Next is a big surpirse for many people. Get 3 zeals. THIS IS NOT A TROLL BUILD Soraka with boots and zeals become amazingly fast, making you able to get in there, throw a few heal, silnce, meteor, and get out. With the ghost I doubt you will be caught unless you are fighting against someone with a really heavy cc team. Your heal is the 1st thing you max out, so early and early-mid game won't be too much of a hassle without ap.
Now you need to grab a deathcap; fast. With this item, your heals will be GODLY.
After your deathcap, you want to finish those zeals Phantom Dancers. The increased movement speed will only add to your fun and annoyance to your enemies.
People may be wondering, "Wait that only fills 5 spaces. What about the 6th?" Well you are saving this for wards. remember that you are a support. It will be really intense, going in and out of the enemy line, healing, silencing, and smashing people in the face with your meteor, BUT you are a SUPPORT.

You want to switch off between heal (q) and infuse (e). Get 1 on meteor (q) as it will be useful in team fights. Most soraka player forget about q as it does very little damage and you max it out last after heal and infuse, but it is a hidden treasure cove. It shreds magic resistance with every hit, and it stacks. Many sorakas are scared to go in the middle of a teamfight and q, but with this build, you will be able to move in and out. Just dont initiate with her. Remember, cc is your worst enemy.

I usually get teleport and ghost. With speed from items and ghost, I can fly to allies and help, and with teleport, even batter. If you are not feeling safe, grab an exhaust as soraka will be ver, very, VERY squishy. you will die be fore you can even blink. Of course you have your nuke heal, so as long as you dont get ganked with cc such as stun, you won't be in too much of a problem.
I recommend teleport and ghost, with exhaust as an option.

That's it guys. Thanks for readin. Hope you have fun playing intense support.
With this build you'll have to pay alot of attention as you won't be lame support from the back line, you'll be in the mid lines maybe even front after all the cc is gone.

AND TO THE PEOPLE WHO THINK THIS IS A TROLL BUILD,
ONCE AGAIN THIS IS NOT A TROLL. LIKE LITERALLY AND SERIOUSLY.
TRY IT OUT BEFORE YOU MARK IT AS A TROLL