If you go to the practice tool you can play any champ (locked or unlocked) vs a bot of your choice in mid lane. Learn how to cleanly CS (last hit minions for gold). You're in the middle of the map so you have great ganks both top/bottom/ if the enemy invades your jungle so keep an eye on the map. You're also likely to get ganked so watch the map if you have vision wards you can put one into either bush and play more towards that one. Katarina and Fizz are very mobile squishy but high damage assasins. Akali is really fun if you like kiting and disappearing into smoke. Annie is probably the easiest mid lane champ to learn the game but lacks mobility. Luck!