For this build, ability wise:

Q: Slows + heals you a bit
W: Heals your team mates
E: Silences & can root.
R: Lots of heal for all team mates instantly.

When using this Soraka build. You want to max heal first as you will be in the backline. Next, you want to max E. As it's incredibly useful in team fights. For example, when there is a team fight happening-- and a couple of enemies bunched up together. You can prevent them from using any spells by silencing as well as have a chance of rooting them. More than often, the combination between your E + W is a major deciding factor for winning your team fights.

Pros:

Great healer in the game
Provides great CC
Extremely strong mid-late game
One of the easier champs to pick up and play
Global ULT

Cons:

Squishy
Team Dependent

VS Comp 1: not that much all in poke (i.e. YUMMI/ NAMI / JANNA/ ETC)
Take.. HEAL + SHIELD rune set.

VS Comp 2: lots of all in poke (i.e. BRAND / THRESH / PYKE/ ETC)
Take.. VS POKE LANE (AKA BRAND) rune set.

VS Comp 3: Contains bulldoser type champs that can rail through frontline to get to backline such as MUNDO/OLAF/ETC.
Take.. SPEEED DEMON rune set.

Boots

Ionian Boots of Lucidity-Provides good amount of CD
Mobility Boots-Great for roaming
Mercury's Treads-Good VS AP heavy comps
Plated Steelcaps-Good VS AD heavy comps


Items

Mikael's Blessing-Provides 20%+ heal power + 20 AH + CC remove for your team mates.
Redemption-Provides 20%+ heal power + 200 HP + area heal for your team
Vigilant Wardstone-Provides 40AH! + 10% movement + more ward storage (stealth+control).


Items (Situational)

Ardent Censer-60AP+ 10 heal power + buff attackers with heals passive.
Staff of Flowing Water-50AP+ 10 heal power + grant team movement w/ heals passive.
Chemtech Putrifier-20 AH + 50AP + grant team mates anti heal per heal.


Resistance

Frozen Heart-20AH+70ARMOR+400MANA+ Damage/AS reduction passive.
Spirit Visage-10AH+450HP+40MR + 25%+ SELF HEAL THROUGH ULT.

Heal Great combination for this Soraka build.

Due to the heal item's later in the game, heal will have a much larger effect on Soraka.
Nimbus cloak + heal movement combined will give you a very nice movement speed boost to help you escape any ganks or unwanted situations.

Barrier survive bursty matchups.

Ignite take when VS heal comps such as YUMMI / SERAPHINE / SENNA.

Exhaust take when VS assassin comps such as ZED / TALON/ ETC.

Warding is one of the most important aspects of the game! Since Soraka is pretty squishy, you do not want to wander around river alone. Though, you may if you have enough information from the map! Always keep an eye on the map and try to locate where the enemy Jungler is. If he's ganking top while the enemy mid is well.. is at mid while your lane is pushed. Go ahead and roam and try to get DEEP wards down! Below are some general hotspots to have wards to get early information on foot traffic.

One tip I always make a habit is to make sure to get 2X control wards each back no matter what!

Now to warding:

Early red side

Early blue side
(you have an option to put pink in pixel ward or in front of drag.. though they have a higher risk of being easily taken down

Drag red side

Drag blue side

Rift/ Baron red side

Rift/ Baron blue side

Thanks for taking a look <3