I get flamed a lot for having the most passive playstyle people have ever seen during the lane phase in high diamond elo, but that doesn't mean it's bad. Sona is by far the hardest scaling support there is. Your only job is to get to that point. I see enemy supports forcing themselves to play aggressive and it just makes them take disadvantageous trades since I'm conserving mana for W and constantly trading inside minions waves with auto attacks. Just make sure you're sustaining your Carry, helping him cs under tower if needed, help get level 2 first (level 1 Q is very good for this) and other things to ensure you meet the most important need to be successful with Sona, not dying and being able to scale into the mid-lategame.

With sona, the most important thing is not to land that nasty 5 man ult. While that's nice and all, your uptime in fights is BY FAR the most important thing since all of your abilities have extremely low cooldown via itemization and the passive cooldown reduction from your ult. Just try to make it so you get 5 man shields on your team, spam heal on cooldown, try to help kill the front line(if you aim the backline you're an easy target. Play next to your adc or midlaner or you will die.), and try to be consistent with your ults by ulting for your teams safety or supporting your teams initial engage. Try to avoid being the primary engage, it's too risky if you die first. Spamming Q in teamfights will make you stack your athenes very well, and when you're at max cdr level 16, your w has a 3.6 second cooldown so you can constantly get very strong heals off with your athenes.

-Your teammates auto attacks on towers get empowered by your q, and yours too. Your powerchord also shreds towers as well.

-When you do the baron with sona, not only is it fast, but you also take 0 damage because of how strong your AOE shields are. Try to control vision around it so you can make so nice baron calls. I typically tell my team that you don't take damage with sona from baron and they won't hesitate to force it when possible.

-dragon fights are super good for you since you can aoe shield multiple people at an early stage which is actually extremely strong. if your team is near your, it will be as if the enemy if trying to kill 4 tanks. Make sure you're always by as many teammates as possible at once.

-Tell your team to stay close, and be a leader. Tell them when you're really strong. Your aoe shields on a 3 second cooldown win teamfights hard. Split pushing isn't that great when sona empowers 5v5's by a very large amount.

Focus on being as consistent as possible. don't do anything flashy. You will gain elo, no matter where you're playing from, don't let the flame get to you, stay positive, and reach your goals.