Sona is an excellent support. Her passive is good to do damage, so as her Q. Her W is great to heal. Her E is excellent to chase enemies. Her ult is great to stun enemies in team fight.

Early Game is the perfect time to counter Sona, incapacitating her to AP or ward well at mid/late game.
How :
1) Her W wastes a lot of mana and don't heal a lot at the early game.
Focus the ADC, making Sona use her W. When she's out of mana, ask for a gank and take out the ADC with no problems.
2) Her Q don't do a lot of damage on the early game, and is her only weapon, instead of her ult.
Sona don't have damage, so her only weapon is her Q and R. Until she reaches level 6, stay aware for her Q; when she uses it, deal the maximum of damage at her, and make her use the W/E, and follow the steps of item 1.
3) She has the life of a minion at the start.
Yes, she have about 500 hp on the start. Depending on the ADC, you can focus her, stopping her from leveling or protecting.

1) She will be doind the build now.
Be aware for her build : she can support so good that she can solo a tank (if well builded/played). Try to counter it the maximum you can.
2) Her R.
She can stun your team decently now. In team fights, don't stay like a line, or else she will stun your whole team!
3) Her E.
Her E is good now. She will use it if bottom lane get ganked. Stun/Slow abilities could be useful now, as it can counter her E. Try to surround her.
4) Her Q.
She got a pretty good poke on Q, so stealth atacks could be usefull.
5) Her W.
If someone of her team is losing life, she will use her W, and that's not good for you. Try to push someone, like the ADC to the middle of your team. If she comes to heal him, stun her, and killthe ADC.

Now she is a hell of a support. She will deal damage with her Q, heal with her W, pursue with her E, use her R just to kill one guy. She is dangerous now.
1) She completed the AP build ?
Now her Q can be a dangerous poke skill damage. Stay away from her, and try to atack her with 2+ teammates, for her Q can atack 2 champions/minions.
2) Who is with her can be cured completely.
If she is at top lane, helping on a teamfight, and the ADC is alone, focus the ADC. He don't got the support now, he will die.
3) Team Fights
Her stun will be useful. She will focus anybody on your team, stun him, and call the team to atack him. When she uses her R, she gets vulnerable : call your team and feed someone with her.