Soraka is, in my opinion, one of the best supports. She has a MR (Magic resist) debuff for your team's mages to deal more damage. She has a heal which increases defences. She has a Mana healer/Silencer. And her really awesome global heal.

Pros:
Has heal
Has global heal
Has Silence
Has Mana recovery
Has MR debuff
If Soraka is ignored she'll become very annoying

Cons:
Long CD heal
Usually focused
If caught out of position Soraka will either die or not be able to support her teammates.

Always start with a faerie charm, 3 pots and 3 wards. This is usually the standard for every support. The only reason you should go back to base is if you're out of wards. She has a mana recover and heal for a reason. Then get Philosopher Stone, good mana and health regen plus gold per 10. Boots also needed to help heal your buddy (What I mean is to be able to reach them in time before they die and when they need your heal immediately). Heart of Gold gives good hp and gold per 10. Kindlegem helps with CDR and more health in which your heal is about 20 sec cd which is clearly way too long so CDR is really needed. The first big item you should get is Shurelya's Revivere, good health and mana regen, CDR auras and a speed boost which is great for chasing, running. Get Aegis of legion next, it makes your team slightly more tanky which helps greatly in team fights. Quicksilver Sash is pretty useful, if you get stunned, snared, silenced use it so you can get back into the fight to heal and silence. Whenever you think your enemy team seems like they always know where you are, buy an oracle. Also have about 2 wards in your inventory at all times. Can really help if you're doing baron, dragon, getting buff or just map awareness. P.S Map awareness is the biggest reason why people die everyday in LoL.

Early game is the time for your laning partner to farm like Chuck Testa. Keep him/her healed and full of mana. Place wards near dragon and surrounding bushes around your lane. CV red buff so if it is gone you can tell if your enemy jungler is ready to gank and warn your allies. Also keep an eye out on the left hand side of your screen where it shows the HPs of your allies then wisely use your ulti to save'em. When you see an enemy die check the minimap and where your ally is. If the ally is about to die fromt he last tower hit use it before it hits, if they are ignited use it. Another reason to use your heal is to get more assists. If my team is in a team fight and I'm not there but I know that team fight is secured i immediately ulti when my teammates are at about 3 quarters health. I know it may sound stupid and worthless but who knows could be saving someone. Also keep using your Promote, when it kills a minion you get gold for it so it's a good way of getting gold stealing your friend's farm without them noticing. If your laning partner is going back heal the Promoted minion when about half hp. I always did this and the enemy would take about more than 5 seconds wasting time on it while they should be going for the other minions.

This is where you really annoy your opponents. You should heal your carry like whenever you can. First at the start use your heal on the tank so he/she becomes heaps more tankier for the beginning when all your opponents attempt to get through to the other allies. Use your silence on the enemy's AP carry. When you see your team all at about half hp use your ulti. If you have been silenced, stunned, snared etc immediately use your Quicksilver Sash. When you know your team has no chance of use your Shurelya's Reviere so you can run away. At the beginning of a team fight dont get into an awkward position. Stay behind your teammates never go to the front.

This is my guide on Soraka. Use it in ranked games mostly. Plz comment to tell me what I should add and what I should tweak. Thank you.