There is a number of things you can do.
first, if you can find some friends to play with. JOining a teamspeak community can help. When you are playing with them don't be afraid to ask them to let you play a role you don't spend much time playing. Hopefully they'll work with you and you can get out of the support role. Note this isn't calling "fill" its a case of saying up front. "Hey can I mid for today. I want to practice."
Second, look at a champ and if you like them try to spam them for a week or two. I only started in January, but I have 60+ games on Jinx (she carried me to level 20) almost 100 on Sejuani (she carried me until I entered ranked play.) and nearly 80 on Sona (I switched to her because Sejuani was and is a must ban in ranked.) I do get my preferred role a lot more because I prefer support, but if your teammates know you have a champ you like to play they'll usually work with you to allow you to play your favorite role. (half the reason I ended up playing Sejuani so much was at the time my regular group had a support main so I had to move somewhere.)
Third, On jungling. Jungling is as much a game of knowing routes and timing as it is about being expert on a champ. You mostly need to know what order to take camps and when they respawn so you can maximize gold while still providing map/lane pressure. Many people recommend Warwick or Yi as starter junglers, but I'd recommend Amumu. He's easy to learn to play as he has only 1 skill shot, and once you upgrade his jungle item he can clear camps effectively. at bronze/silver Elos he's quite effective as a ganker as well. The pattern is easy, Put a point into W and then make sure you have a leash. Smite, turn on W and kill Gromp. Put a point into Q and then Q to the blue buff and kill it. (you'll use 1-2 potions here depending on how good your leash was.) turn of W while you walk over to the red buff, Turn it back on a and then q to the red buff using smite to finish it. Then depending on your health you can gank if you feel strong, kill a river scuttle for gold or back and upgrade your jungle item. After that, its just a case of making sure your buffs are never up long while you clear camps and visit lanes.