Starting champions. You really can't beat the current 450's. Many of them are specifically designed to teach you the roles of the game. The best teacher, in my opinion, is Ashe.
ADC is a straightforward role with little nuance until higher ELO. Ashe's kit is designed to make you think without punishing you for not thinking. She gets bonus gold for last-hitting, so she teaches players to last hit. She gets free crits for biding her time and waiting for the right moment to attack (also for last hitting and not constantly attacking), she has a ranged vision ability which teaches you how important vision is so you learn to buy wards, she has both soft and hard CC, she has no complicated mechanics or movement abilities to confuse new players.
For Support, I recommend a healer. I think Soraka is a 450, but she has more complicated mechanics now than she used to. I like Sona for beginners, but she's a 3150.
For mages, I recommend Annie or Ryze.
For top laners, I recommend Garen.
Jungling in AI isn't great. It leaves your top-laner to fight 2 all-in AI alone. Also, in my experience, people only play jungle in AI so they can enter a lane that doesn't need help under the guise of a "gank" and then just steal the kills.
I'm not saying don't practice Jungle; go fight the camps and learn their patterns and strengths. Just put more of your focus on laning, team work, and learning ability interactions with champs.
Runes...I keep saying I'm going to buy more runes, and then I can't decide what to get. I would say once you have 10 champions you can start saving for Runes. 10 champions allows you to play ARAM which is a normal mode game that let's you practice team fighting and (forces) you to play champs you haven't played before or much.