-Early lane phase: its pretty much common sense to harass the enemy safely with your Q while making sure you and your ADC SUSTAIN hp in lane with your W, the reason for my mastery choice is with the mastery Pick Pocket and the Gold per second gain from building it to the second level for nothing, when you auto attack them you get 3- 24 Gold when landing a good Q. In solo Que be sure to let your ADC know your harassing for gold income NOT because you want him to go in for the kill. Usually turns out after poking them with enough Q's and Sona passives you will wind up going for kills anyway. But its best to play safe until your level 6.
-Side note: Sona's Ultimate and sustain Syncs up REALLY well with vayne's Condemn. Most cases where Vayne is countered It wouldn't really matter if you land the condemn and Ult them after. 9/10 times guarantees you at least 1 kill if not both!
- Starting items: 2 vision wards, Vision trinket and Frost dagger, why? Gold per second keeps you helpful and the warding is still your job every time you B you should be able to afford and keep at least 1 or 2 wards while continually getting the items for your build. Neat Right?
- where should i always keep warded?: I tend to ward Right in front of dragon and their bush or ours, Depending on who is pushed back them or us? If you get extra money for an extra ward i recommend warding their blue buff for that extra heads up.
- Why No boots you noob?: Sona REALLY does not need boots to escape honestly, late game especially. Sure you could get cooldown but its really not needed totally. ( you could just change ap runes to cooldown reduction) plus i don't know if its just me, but i find boots to be a double edged sword.
Why? Because sometimes the ability to get greedy and chase someone isn't the best idea, and two, because i feel it takes away from your true potential to learn how to escape from a bad situation, If you can escape with no boots from 2-5 people whamming on you, you can DEFINATELY do it with them. *Plus later on the rylai's is there to slow pursuers ;)*

Why not magic pene?: Thats really only valuable early game from my understanding when no one has MR. Besides all you want to do is maximize your heal, and your movement speed buff. those are far more important tools as support.

What is your prefered build order?: I can't tell you honestly, my play style is for me find what works for you. i recommend lich bane or rabadons.

Sona mid game... oh how she shines, by the 20 minute marker any amateur sona can at least get a lich bane and rabadone's cap. Those two items alone can help chunk down the squishy support, adc, or mid lane. Basically at this point that's your job. Stun in team fights, Melt the squishy people with your Q combo with the lich bane, and use your E for making minions push faster or helping team mates escape or chase down people. Simple right? And of Course Keep them wards in river by dragon and baron as much as you can. Doing these simple things can keep you and your team mates alive and happy, So you don't hear, "WHY U NO WARD?!, or Omg sona UNINSTALL PLZ" :|

Yep this is where sona is ******ed. Keep warding where u should and feel is needed. and at this point.... Your basically like an AP carry. Your Q with a lich bane and power chord can either 1 shot a adc or mid lane character or bring them to their knees for a team mate to finish it.
Your movement speed boost can help push lanes fast, chase down low hp enemies OR disengage a enemy chasing your teammates. Sweet right? After u hit all 6 items and are wonder what can i do to break 900+ ap, if the game gets that far, you can sell your Frost Dagger for another Death fire for Cooldown or another Zhonia hourglass for the bonus armor. *either way you get 120 AP* :D
Well Good luck guys this is all i can think about to touch bases on.