When do you pick Sona?

The "Sona Main" answer: Always

The "not Sona Main" answer: Generally when the other team also picks another squishy support (Janna, Soraka, Nami, etc) because Sona goes pretty even with them, or if the majority of your team would benefit the most from Sona (ex. if you're going the Ardent Censor build, that item goes well with champions that are AA focused)

When not to play Sona: Typical counters you would expect are Thresh, Leona, and Blitz because they all fuck up your life. However I only reconsider playing Sona into Blitz, because his hook has some janky ass animations and he alone can one shot you. Thresh's hook is slower and easier to dodge imo, and Leona can easily be outplayed by going the healer route and staying out of her range (Leona's E has 25 more range than Sona's Q, but Sona's Q always has a shorter cd. It just takes for Leona to miss her E for Sona to get her poke off safely)

In most situations, but I main her so I'm not objective.

In low elo I like her a lot, she provides a lot of poke, great utility and is fun to play. I'm sure there are better picks and the higher elo you are then she doesn't (I imagine) fit every team comp.

she has good poke and good sustain and that makes her a good pick in most situations. I personally don't find her too fun cuz the longer the game goes the more I feel like I'm just smashing my face on my keyboard, but, that being said, there are definitely more boring champs out there to play and she is in a pretty good spot right now with all the support item changes that happened last patch. I'm surprised she's not played more tbh

Honestly, you pick Sona until you either get good with Soraka or get enough IP for Nami (Who does basically the same stuff, only her heal bounces and does damage, her ult is a lane wide knock-up versus a 200 unit snare, and she has a useful buff and snare in her e and q that Sona doesn't get. Plus her passive is to give move speed whenever she uses and ability on an ally or herself, so there goes Sona's E...)

You can pick her pretty much whenever you want, but keep in mind you're super squishy. A lot of match-ups can one-shot you if they catch you, so, well... don't get caught: Blitz, Thresh, Leona, Brand, and if the enemy ADC has any CC of their own, even a Nami bubble can be enough.

Against an enemy lane that can out-poke your own blue poke, max green first. If you're in a position to be aggressive, max blue. Spellthief is usually better, but Coin has its situational uses. Don't go for an AP build with Lichbane and/or Tear, it's a noob trap. And obviously Windspeaker keystone, not Thunderlord. Your build should include a situational selection of all Forbidden Idol items, Athene, Solari, and maybe a defensive item (Frozen Heart or something). Special note for Ardent Censer if you have an heavily aa-based team, your green AoE can proc it with 100% uptime on your full team.

Don't hesitate to ask if you want more details.

EDIT: one big problem is if your team has no frontline, in that case you'd be better off picking a Tanky Support yourself, but you can often convince your Top and/or Jungle to fulfill that role for you.

aoe and lategame utility carry. just don't ever leave your team or try to 1v1, you will die. good in protect and wombo comps.

(gotta be carefull with annie though)are all champions who in the support role get beaten in laning phase especially if you lane with lets say or a decent

Sona is good to win lane since a sona should win pretty much every lane unless the enemy jungler pressures or some champion is just op or you get all-inned early and get killed.

I also recently found sona is good against burst champs like katarina since your passive interaction with W makes enemies do 25% less damage. (this percent can be insreased with AP and that's why AP on Sona support is somewhat viable as along with higher heals you reduce enemy burst damage more.)

Sona's passive interaction with her E slows enemy units

and Sona's Q does more extra bonus damage on auto.

Really the differnce between a bad Sona and a good Sona is probably mostly on the fact a good Sona realizes that choosing when to auto and what passive to use actually makes a big difference.

Sona used to be considered the best support to play with vayne and it also used to be considered her greatest counter pick since you could just repeatadly Q + auto her when she goes for creeps and vayne has a hard time all-inning sona early and doesn't do enough burst damage to make damage stick.