Now that you have decided to play Soraka there is some things you need to keep in mind: Your health is the most vital resource to you. You use it to heal and block damage for your ADC, HOWEVER you should never put yourself in a situation in which you have no mana to heal, no health to heal, or you just die.
As a support it's your job to keep your ADC alive while simultaneously keeping yourself alive and when you get your ult, everyone else alive. You need to keep in mind that you will be flamed in some games for not going in when the fight was clearly against you and your ADC. Simply ignore it and help your ADC get caught back up on CS.
Now, as Soraka you are a late game champion so highly mobile early game champions that can snowball are the best bet for an ADC. Some good picks are:
Ezreal, Vayne, Draven, Miss Fortune, Anivia, Yasuo, Volibear, Ashe, Varus, and Fizz.
I know these picks aren't all exactly meta but try it out, it's really fun to make Anivia ADC work and it's hilarious once the ball gets rolling on all of the champions.
Now that you have an ADC It's time to determine how you should play based on what your ADC's play style is, what your enemy laners are, and how your team is going to play.
(EX: Draven on enemy team with a Lulu; Talisman would be best for that situation to run away faster EXC)

Now that you have chosen your play style it's time to play. In laning phase it's your job to zone your opponents, poke down minions for your ADC to last hit, ward, heal, and snare.
If you're getting low on health land your Q on an enemy, this steals life from them and heals you as well as giving you a speed boost.
If the enemy is trying to engage predict their movements and throw your E down to snare them the second time it bursts.
If you're being ganked land a Q on an enemy, heal your ADC (this transfers the speed buff and the health regeneration) and get them back to tower as fast as possible.
Another good rule of thumb is to never stop moving and punish their ADC for going out of line and messing up position.

At around 20 minutes the enemy will grow tired of your team and start to group. When this happens make sure to ping, group, and take objectives. NEVER split push in this phase you WILL get picked off.
If a team fight breaks out, stay in the back line, heal the tanks, and attack whatever goes into your range to give them health regeneration. Remember to stay safe while simultaneously babysitting your whole team's health bars.
By now your W should be maxed making it have a 1 second cooldown; make sure to abuse this if the enemy assassin tries to pick off your ADC.

In this phase you're job is to stay by the ADC and keep him alive. Group whenever an enemy is spotted and be very careful when you and your ADC are split pushing. Take whatever objective you wanted then back to base. NEVER stay out too long otherwise the enemy team will find you and group around to destroy the two squishy players taking objectives from them.
Again just stay safe and run towards anyone that has your passive on them as you can heal them often before they get bursted down causing them to win whatever fight they where in.

Your are now roaming to whoever needs the most help with objectives. If you did your job correctly your ADC should be dishing out a lot of damage but can still get picked off easily so make sure they stay within a lane's width of you. take everything you want; but push inhibs and towers as the first priority.
Keep your tanks safe, use your ult to heal allies across the map, and lifesteal people to death.
Have fun!