For my build, I use those Runes:
Quint: I take AP on quint for damage.(15 Ap, Most used)
Glyphs: I take MR because if you go Mid, you'll encounter A lot of Ap carries, and you need the MR Obviously.
Marks: I take Magic Pen for 2 reasons:
1)It is needed by a champions that deals constant damage with a spell,
2)Magic Pen stacks really well with her Q.
Seals:I take armor seals to become more durable against ADCS and Bruisers.

Offense:
I take 21 Points In offense for a standard Ap caster, taking CDR,really important on Soraka,Spell Weaving(Bananas Op)and the other Damage increase masteries.
Utility
I'd like to take 9 Points in Utility for some reasons:
The biscuits, that gives a lot of sustain in lane, The movement speed, which is really good and the mana/health regen.

For Items I have a lot of Options:
As A starting item, Get a Doran's Ring and 2 Health Potion which they will be turned into the Biscuits thanks to the masteries. ( It's pretty standard)
There's also another start, with the Faerie Charm if you really want to spam in lane, but I don't very like it.
Then First Very Important Item is Anthene's Unholy Grail or the Tear of the Goddess ( If you want to spam)
The next Important Buy is Rylai's Crystal Scepter which is really good in Teamfight and chasing the enemies with starcall. Also it gives you 500 Hp for more durability.
The start to make a Rabadon's Deathcap Immediatly after, Which gives you a lot of Ap.
After you bought those 3 items, start to look at the other lanes:
1) If the Enemy ADC is feed build a Frozen Heart
2) If your enemy mid laner is fed or deals lots of damage, Build a Spirit Visage which increases also your healing on yourself.
Take Rod of Ages if you need more durability and Mana.

For Summoner spells, I have some Options:

Flash and Ignite Works well, with flash you can escape or reposition and with Ignite you can take down your opponent.
Flash and Barrier is another option, wich gives you a shield that let you survive in team fight or in lane.
Flash and Teleport this is what i don't like to use.

But When You pick Soraka?

Well, I usually pick Soraka for fun in lane and nice farm, but i actually Pick her against:
- Which can be stopped easly by the silence. If she ults, you silence her.
- Thanks to your armor buff, you can decrease the damage he does on you while fighting him. Also Your silence can just stop him dashing trough your minions. Also his shield is useless.
- ] Same for Yasuo. You can also Silence him while he casts his ult, so he can't turn invis. ( but still you hit him with starcall.)

I DON'T Pick Soraka Against:
- If it's An Ap Tristana, you're healing is reduced by her E. If she ever jumps to harass you, silence her immediatly.
- Swain is really badass, he has a good sustain with his ult too, and if he combos you from afar, you're dead.
- That's not real a counter, if you can stack your Q, you can win easly, but beware if he builds lots of MR.

Pros:
- Great Sustain in lane with her W.
- Useful silence to stop champions Like Katarina's ult or Malz's ult.
- Ult really Useful to survive in lane and help other lanes.
- Nice Farm with Her Q.
- Bananas Op
Cons:
- Really Squishy early.
- Need to Build some hp on her. ( Rylai)
- No escapes.
- Not that aggressive in lane.
- Horn on da head

Farming with Soraka is quite Easy.

EARLY GAME:
At level one try just to last hit with basic attacks until lv 3.
From there start to last hit minions with your Q, but be careful: Don't go too far ahead, or you can get ganked easly.
MID GAME:
Now that you have All 5 points on your Q, you can really farm fast creeps. You'll get a lot of golds.

In end, is a really nice champion,which can support your allies and win trough the teamfight.

Her silence is a really good spell if casted on a engaging It can prevent him to use his ult and let your teamates get away from him.
I really like Soraka for it's style and story. Hope you like my first guide!