Bonus mana allowys me to spam more abillites.
Mana regen also.
Ability power is important especially at lower levels.Becouse your heal gonna heal for like 35 withoun ap runes.
Sonais really champion that is based on level so i take "More Xp Quitessences" for faster leveling.

In offense i just take Ability power.
In defense Magic resist and Armor which helps me survive enemy focuses on me.
I take typical support masteries in utility,More XP,Less dieing time,Health regen,Mana regen,Extra movement coldown reduction and summoner spells coldown reduction.

Shurleyas Reverie is great support item for your team so you take it after boots.
Ionian Boots+Morelios Evil Tome+Intelligence (3) in mastery tree gives you great coldown redcution and helps you keep all of your 3 auras activated.Ryalis Crystal Scepeter.It helps me chase down enemies and great when combinizing it with Shurleyas Reverie and Randulin's Omen.Great chasedown combination.Zhryonas Hourglass great ap item and item for surviving

I take Hymn of Valor at level 1 which gives us chance to get first blood in our lane also i need it rank 1 for my Power Chord,Each time i have it ready i take Hymn of Valor aura and attack with Power Chord.Aria of Preservence is my main support spell so i max it first.Song of Celerity is also good support spell so i max it when i cant max Cresendo or Aria of Preservence.

Those 2 spells help me stay alive in teamfights and take off enemy aggro.