Why am I bad at everything but TOP. I need help!

Hey everyone. I'm level 13 and got the game a week ago. I absolutely love playing TOP and I have a very high win rate with Garen. I also have Pantheon and Darius as back ups in case of Garen ban. Other champion I have is LEONA which I kinda regret because she's trash (She's probably good but I don't know how to play her). Anyways, I know you have to select two positions. I always choose TOP first and majority of the time i get it, But whenever I don't get TOP I'm just so bad. I don't know how to play support or jungle or mid. I seriously just don't know who to play or what to do. If I am good at top and love offensive characters, who should I buy and what should I play as a back up to top? I heard maybe I could Pantheon Jungle or Darius Mid which would be awesome because I love Darius but I need some advice. It's really frustrating when I don't get TOP and I feel like I'm letting down my team.