assist by teqnik41, on Flickr

The screen shot of the score says it all!
The maven of assists, nobody helps out in group fights better than Sona. With this build you will be giving cooldown reduction, armor/spell resist, speed(lots of it), and ability power buff. On top of that you will be earning good amount of gold over time.
This is a purely support build, but you will also be doing some decent damage once you start acquiring some mejai's stacks. Those stacks will also be boosted by rabadon's deathcap, in effect making you a healing, taxi-ing, sexy assist machine!

First item I try to go for is the philosophers-stone as it will give you mana regen and gold over time. I usually go for faerie gem first for the mana regen. Stay back and always lane with someone, never stay alone unless you are by a tower and even then watch out for high burst dps like Akali/Xin. keep your Aria of Perseverance up for your lane mate and cast it consistently, even if your mate is rather high on health, so you can get your 3 spell cast damage buff to use on harassing a champ.
Next up get kages-lucky-pick and you will have gold ticking every second on your screen with no pause. Keep playing defensively and never be on the front line.
Boots-of-speed is what you want to get either after kage's or before, its up to you. Boots as always are necessary and build them up to boots-of-swiftness in the future so you can either escape or run into a battle to help allies.
mejais-soulstealer is amazing for The Maven of Assists as you will be getting stacks after stacks from assists, as long as you play defensively, you will max out the stacks quickly. This means better heals and more harrasing damage in group fights.
shurelyas-reverie this item is next as it is AMAZING for Sona. Not only will you get mana regen and cooldown reduction, but you also get health/regen. The real kicker is the active which will save your teammates 90% of the time if you time it right. Conversely, it will help them chase down an enemy!. This along with your Song of Celerity will let you stay alive as well so keep it on your "2" key slot.
deathfire-grasp and rabadons-deathcap for 2 more actives that will save you and help your team against the big healthies, along with great passives :).
Finally soul-shroud so you can share the cooldown reduction and gain a little more survivability in the late game when the big hits can take you down quickly.

• Use those ACTIVES! you will have 3 of them be aware of their cooldowns.
• Pop Song of Celerity before using Shurelya's active
• Heal often! It's a small heal but eventually with Mejai's stacks it will save your team.
• NEVER LANE ALONE! You are squishy, you will die quick in the early game but will mean death for your attacker later on.
• Once mana regen is high, try to keep all 3 buffs up in a team fight.
• Have Fun knowing you were a key player in getting the WIN for your team!