Please don't play Janna as an AP carry. You can get a little AP, because it does strengthen her shield, but don't go crazy and buy a deathcap. Focus on support items (sight stone, frozen heart, locket, mikael's,etc.) and get CDR. CDR is incredible on Janna because the more shields, and slows, and knockups you can pump out the better. One second off your shield can mean the difference between saving your ADC's life or watching them die as you sit there helpless.
In trades I always just focus the enemy ADC (unless the other support is very dangerous like Annie or something). Ideally you want to immediately shield your ADC, knock up both aggressors, and W their ADC as well. Then just start autoing the enemy adc. I don't think people realize the damage you can do with just autos even as someone like janna.
Janna is best with hard-hitting ADCs like Draven, Vayne and Lucian because her Shield increases their AD and thus makes them hit even HARDER. The damage can become insane once you both get a few items.
In teamfights you want to focus on helping and protecting your ADC. Your ult is crazy powerful and use it to stop assassins/fighters from blowing up your ADC. honestly i feel like this is the main reason Janna is so strong because she counters a lot of the "dive in and kill the ADC in 2 seconds" bullshit that many team comps in solo q depend on.
I think what separates a good Janna from a bad one is being able to shield poke. If you can anticipate and shield poke you will me your lane a lot easier. It's harder than it sounds. If you shield too early enemies will simply wiat for it to end and you've just wasted a lot of mana for nothing. Janna does not have a heal pre-6 shielding is the only way to keep your ADC healthy before an all-in. Also make sure to poke the enemy ADC. You have your Frostfang with makes your autos do extra damage. Make sure to use those charges. You can run up, shield yourself, and do some autos that will be more powerful because of the AD your shield is giving you.
Anyways that's all I got for now. I hope you enjoy Janna I honestly think she is the most fun most pure support in the game and I hope you can figure her out.