Well it depends on what you want to learn.
Xerath and Vel'koz are good for learning skillshot accuracy, since they're basically 100% skill shots.
Orianna is good for learning positioning, as you have to worry about your own positioning as well as the ball.
Lulu, Orianna, and Karma are good for learning how to utilize your skills and items to benefit both yourself as well as your teammates.
Morgana and Annie are pretty good for just learning mid lane in general.
Lissandra, while not technically an immobile mage, is really good for learning how to roam and affect other lanes.
Now the issue is that you don't want to jump straight in on Xerath, Vel'koz, and Orianna, as they're very difficult champions and can be completely useless if not played well. I think Morgana, Annie, and Karma would be the three easiest to start out with if you're unfamiliar with the lane itself.