I'm not the greatest Sona player ever, but I have played 100+ games w/ her and I hope that the following can help you.
You have two good choices to go with for her, AP Nuke or AP Support. First thing you need to find out is who you are laning with and how you are going to benefit them. Are you there to make sure they get fed, or are you there to get fed. Playing her as a Nuke takes some practice to get use to her passive, so i'll start out with support for her. You want to take her heal first if you support, and pretty much let your laning partner get most last hits. Worried about not getting gold, not a problem, get a philosopher's stone, it gives you 5 gold/10 seconds with decent HP and MP regen, plus it turns into a nice support item which gives nearby teammates a speed boost when activated. You should Max your Heal asap, but put 1 point into your other skills, then start maxing it. Max your Speed second, and your Nuke 3rd. Put a point into your ult every chance you get too. Then you should build Merc Treads (for CC/AP heavy teams) or Ionian boots for Cooldown Reduction. Then build a Champions Aura, Rod of Ages, and a Hero's Armor for some auras to help support your team. Basically, w/ the CD reduction and plenty of mana, you can consistantly keep all 3 auras on your teammates, which buffs armor/magic resist, damage, and their movement speed. And make sure you pop your crescendo on enemy champs when they get close.
For AP Nuke Sona, Max Nuke, then Speed, then Heal. First item you should get is a Rod of Ages, then a Lich Bane, then a Deathcap. The lichbane adds all AP to your next attack after you use an ability, and if you get a crescendo proc on your Nuke, with a lich bane hit, you can Basic Attack people for 1000 easily. Just remember an AP Nuke Sona can be burned down quickly, and cannot burn people quite as fast as say Annie or Brand, but i promise a well timed ult with your abilites can do some massive damage and pick you up a bunch of kills. Just remember that you can heal your teammates, so you should do that whenever you can, and try to ult as many people as possible. I hope this helps you. If this is confusing btw, i do apologize, my organization of champs is terrible for the most part