Vi is the strongest and easiest champion on the game currently. You should definitely consider picking her up IMO before she gets nerfed. She is banned a large portion of the time at ~Diamond 5 MMR, no idea about lower ELO though.
Gragas is really strong if you can get going on him. Kayle is currently the most broken mid champ. 0 skillshots and a 2.5 sec invulnerability at max rank can turn teamfights (just build Nashor's, Lich Bane, Dcap, Void, and then last 2 items are usually situational).
Janna isn't a great support imo at the minute if you want to be able to carry. She's generally used in a lot safer lanes whereby you want to do nothing but farm and poke. Consider picking up Thresh, Annie and Leona for support.
Miss Fortune also isn't great, but she can be ok with specific teams. Jinx and Lucian are the best Marksmen at the minute.
Make sure to stick to a couple of champs and focus on mastering at most 2 different roles. You'll learn champions and the game a lot quicker this way.
Pantheon jungle is also ok, but he has pretty slow clear time and kind of a niche pick, but he can definitely work. I'd strongly suggest picking up Vi, Kha'Zix and Elise though for jungle.
Anymore questions, fire away.