Most important advice is to have cookies, and the Clarity summoner spell.
Your powers depend on Mana, without power your of no use to your lane, or team.

As a bonus Clarity gives nearby enemies also Mana, so they can longer stay in the game, without having to go back to base, this will allow for longer duration of your teams attack time. And thus win those fights !

In contrast Exhaust, requires you to be very near an enemy (thats a bad position for sona your not a tank), you can only attack 1 champ with it, and your team will have to visit the shop more often, its not good for team fights at all. And for exhaust to work you need to aim to, which can go wrong.. so DONT EVER use Exhaust as Sona, its only your marksman who bennefits a little, but he doesnt want you dead either so take no risk that nearby the enemy.

First of all, before the game starts, know your enemy, and your marksman.
If your marksman is a rookie (beginner) or simply weak, start as a Healing Sona.

If your champ is a bit stronger, and your enemy not that dangerous start as an attacking Sona. You will then poke your enemy giving your marksman time to farm. In most cases its best to attack their marksman. After first attack start with healing power.
A nice thing of Sona as Poke harassing hero, is that you don't need to aim, you only need to be in distance, and behind your minions your in most cases safe, for quick attacks

Decide upon how the game progress to expand attack or heal power.
And if available expand ult.

One of the last powers to expand is movement speed.

Keep your marksman health at 100% all time.

Use Clarity when his and your (so only if both) are below 50%
In rare cases you would use clarity for your marksman, while you still have enough.

As with all the Mana and Heal you can give, a good start round before shopping would be to have used clarity twice on your marksman, so your bot lane stays much longer in the fight then the enemy team can (they will need to visit shop earlier for health) in teir shopping time, your marksman will have more time to farm and will level up faster.

Dont farm minoins, but once in a while its not bad, you also need to level up, to provider better heal or stronger attack.

Although your a support if you can kill (by poking) just do it.
Some team members might not like you will kill too, well in the end its about group advantage, so its fine to also kill, its better that you kill then you nearby friend team gets more damage.

As a healing Sona you stay behind your marksman (you can late invest boots)
As a poking Sona you might need Magic boots earlier.

What you can do with your ultimate

Ask if your marksman ult is ready, tell him your lead the attack.
Sit hidden in a bush, flash forward to enemy and use ult to stun, followed by an attack.
After that marksman attacks ult too, most likely its a kill.

Look if both your enemies line up nice, if so use ult and stun both.

Ult can be a get away, while you flee too.

Its important that towards the end game you follow your team where ever there is a team fight.
So you will heal them or give them mana, you can also poke in those team fights, but since your weaker your not a frontline atacker.

Its about keeping those who do have attack force healthy
And your solari item, helps them in the team fight.

Towards the end you also start to invest your powers to movement speed, so after a devastating attack, you can as a team flee. Or reach out faster for a counter attack.

With group attacks, if one of your team hero's gets really low on power and flees, assist him and keep him healthy be sure he reaches a turret where he can TP back.