So my friend gifted me Janna and one of her skins. This means I want to pick her up/learn her. I have played her enough and watched some YouTube to understand her. I am just wondering who should I pick her with and who should I pick her against. I know that having Vayne and Janna works well. I know that Janna and Ashe is decent. I am wondering what adc's and supports to pick her against and who to pick her with?
Janna can work fairly well with any ADC, what with her shield that gives them AD and all her disengage, so you can pretty much pick her with every ADC and do fairly well. She generally fits into any comp outside of diving ones, but can still do fairly well there with well placed Qs and shields.
One of the best times to pick her is when the enemy is really engagey, she doesn't deal too well against a pokey comp. So in bot lane you're looking at Leona, Thresh... Pretty much anyone who wants to jump in your face. If the enemy picks something like Varus and Vel'Koz, you're better off going someone with more sustain. Her shield can be used to to stop poke though, so it's not all bad.
Just pick her and play her. Janna is and has always been to good at what she does.
Honestly she's good with any adc since she's such a safe pick. She's even better if you play with someone who has an aggressive playstyle. She's a counter to any melee champs so picking against naut, alistar, leona, or any other support that gets in your face is a good idea. Other than that her shield is really strong-- bonus AD and shield at the same time so I'd suggest playing with ADs that are more auto-attack based (vayne, cait, jinx, kalista).
Janna is a pretty well rounded support who doesn't lose too much if you pair her with an ADC she doesn't "synergise" well with. I usually take Janna with Vayne, Corki, Jinx and Ashe, but again, she's pretty well rounded and a relatively safe pick so don't be nervous if you pick her with someone like Quinn or Kog'Maw.
She does well in general, but against harass supports, she doesn't fare as well because her peel isn't as useful.
So; 
and especially 
all counter her to some effect.
With: 
Not with: 
(or yes unless you have crazy good synergy)
Against: 
and overall a lot of AoE in the other team
Not against: 
other bot lane champs are pretty neutral imo.
Note that for her to do good against , especially the later 2, you need to avoid getting hooked yourself as you can only react when an ally gets hit.
Take her if there is a 
on your team.
