Looking at your profile, you have 300~ wins. You've got about 600 games played overall, which is pretty good. If you played your best every game, you could probably sit at 1-1.2k Elo (unless you're somehow stupidly good, like Owl, in which case you could get higher).
My first piece of advice: Be able to get over a frustrating loss. In ranked, at a lower Elo, you WILL have one of two things happen: One, you won't be good enough to play at the level you're at, and you'll feed. Or two, your team will feed super hard and you won't be able to carry. Either way, you're probably going to be very frustrated. Learn to get past this. It happens, and you need to be able to go into the next game feeling like you did before you started playing.
On to your champion picks:
Mid lane Annie isn't very good nowadays because there are so many people that are just better than her, period.
Miss Fortune is a strong AD carry, but she falls off late game and doesn't have as good a laning phase as a hero like Ezreal or Graves, two top tier ADs.
Zilean is an iffy choice. He doesn't really provide the AD carry with any sustain, and only really has his E for the speed up. On the other hard, his ultimate is REALLY strong. 8 minute buffed version of GA is ridiculous. Leona is a super good support with most ADs.
Sejuani and Jax are both sub par jungle picks. Perhaps look into heroes like Mundo, Lee Sin, etc.
Jax is a good top lane choice, but I'd suggest you pick up a few other ones as well.
Now, this is assuming you're playing against top tier 3k Elo players. At the level you'll be playing at, champion doesn't matter. Skill with that champion does. Scarra is going to stomp a LeBlanc mid as Cassiopeia at 1.2k Elo, simply because Scarra is a significantly better player than the person he'll be against. If you're the best Annie in the world, play Annie, by all means. But against the best of the best, Annie won't be as strong.