Welcome to my first ever Mobafire guide!
When thinking about this initially, I had trouble deciding what champion to do.
I eventually decided on Sona. She was the first champion I bought, I main her, and I've gotten playing her down to a science! For the record, this is NOT a comprehensive guide about how to Sona, this is just a build that works great for me and some tricks that I like to use. Not much else to say here, so without further adieux, let's begin!

Hybrid Pen. is incredibly useful for Sona because it gives her significant bonus damage on her + + combo. (See Unique Skills for details)

Movespeed quints are just because... YOU GOTTA GO FAST!!!

Spellthief's Edge Spellthief's Edge is a very standard starting item for Sona because of the poke reward, and just get as many Mana Potions Mana Potion as you can before the Warding Totem Stealth Ward. Tear of the Goddess Tear of the Goddess is essential for supplying a steady flow of mana for heals and pokes to help your allies. Early Boots Boots are also good; that's all I have to say about them. (Don't forget your Ugg's, kids!) Once you get your Sightstone Sightstone, swap out your totem Stealth Ward for a Sweeping Lens Sweeping Lens. Finding pesky wards is well worth it. Upgrade to an Oracle's Lens Oracle's Lens whenever as soon as it's necessary. Boots of Mobility with Alacrity Mobility Boots make for a huge speed bonus on Sona. With your active, you get around 500 movement speed which is key for running away since Sona is extremely squishy. Ardent Censer Ardent Censer is essential on Sona because it gives your auras even more purpose. The Ardent Censer Ardent Censer also gives you an 8% movement speed boost which makes you even faster! Then get Frostfang Frostfang for better stats, and Sheen Sheen for Spellblade (which is amazing for pokes). Fully upgrade Frostfang Frostfang to Frost Queen's Claim Frost Queen's Claim. The active is very helpful late game with its 80% slow. Manamune Manamune/Muramana Muramana adds to the effectiveness of Spellblade with its added AD. Lich Bane Lich Bane adds to your offensive Super(Sona)Powers in the late game with its great increase in AP while still retaining Spellblade with a nifty little +5% movement speed. Round off this build with the Ruby Sightstone Ruby Sightstone - extra health is never a bad thing.

Get your heal first to reverse the effects of unfriendly pokes, then max your q for incredible poke damage potential.

Masteries are pretty standard for supports in the utility tree, but instead of defense, I like to take caster-related skills in the offense tree.

Flash is a must-have on almost any champion, and Exhaust is to give your adc some more time to get the killing blows, or to protect low health allies from taking too much damage from an opposing mage or adc.

Pros

+ Can be very fast mid-game. + Heals for days. + Abilities can affect multiple allies. + Amazing pokes.

Cons

- Extremely squishy. - Quite slow early game.

Sona's main combo is incredibly useful, especially with teammates. If there are many enemies in one place, and your team is itching to help, cast your Crescendo while trying to stun as many enemies as possible. Then cast your Song of Celerity on your allies to get them in close and cast your Aria of Perseverance to give some heals and a shield to all. Then, your passive will be one spell away from activating, so cast your Hymn of Valor and immediately follow up with a basic attack to deal the bonus Power Chord damage plus the added damage from your Hymn of Valor . When in lane, whenever you see your passive has 2 stacks, cast your Hymn of Valor and immediately follow up with a basic attack to poke the enemy for a good chunk of health while gaining around 20-30 gold from Spellthief's Edge Spellthief's Edge/Frostfang Frostfang/Frost Queen's Claim's Frost Queen's Claim passive.