Try at the start play a bit aggressive because u deal very much dmg in the early game. U are maybe the support with most dmg early, but if u are against a Thresh og Blitzcrank try not to be so much aggressive, because u just get pulled and u die because u are so squize. when u start with your "Q" try go get your passive (use 3 abilitys to get passive) and not attack the buffs your jungler start with because then u lose it. In lane get a autoattack with passive and a "Q" the u deal much dmg.

When u hit lvl 6 u wonna have your enemies pretty low so u can ult them and your adc can just get a easy double kill. But if your against a Twisted Fate dont use ult as much because u shall use it to stun him when he ganks bot.

When u are Sona and u are in a team fight u wanna stun a many as much. So wait with your ult to stun as many as much. But not to soon or not to early, because if u use it to early u maybe not stun so many, and if u stun to soon there ae some of your team mates there are dead.

I use the Flash because I like to flash ulti and just stun the hole team and then my one team just can come and kill them.

I use egnite for max dmg so u can kill your enemies fast, and probely dont have to have Exhasut beacuse u have a movement speed buf to the hole team adn u have a massive stun

Shurelya's Reverie: I like to catch up to enemies or run away from fights and give my team a movemant speed buuf in the team fights, that gives my team more mobilty so they just can run all the way up to the carry, get some mana and HP reagen so i can (in lane) heal my mate and just heal my self with that P. stone
Ionian Boots of Lucidity: I get them because i wanna heal and do as much dmg as possible in team fights.
Runic Bulwark: I get that item because i wanna make my team more tanky in team fights.
Sight Stone: As a support u shall ward the map, and the sight stone does that good, u get 5 wards and can place 3 of them on the map.
Locket of the Iron Solari: I will get my team kmore tanky/give them a shield up to 100-200 dmg.
Shard of True Ice: I get Shard of true ice because i wanna get more AP so i heal more and if the enemy have much mobility i get it so i slow them.
Oracle: All supports shall get Oracle so they can she the enemy wards and clear them

I run 0-9-21.

I take 0 ponits in Offense because in am not an adc or mid or top or jungler i am a support and i shall help me mates with heal and those kind if stuff. I could had did it for the CD but i do not because i have all the CD i need in the build and ing the Utility.
I take the 9 points in Defense because Sona is realy squize in the early game.
I take the 21 points in Utility because it gives me more mana reagen and it gives me more gold at the start and it gives me lower CD to almost everything.

U always need to buy an Oracle as a support because u need to see the enemies wards and then denie the enemies vision.