I have a Torker Unistar with a 20" wheel and that's been perfect to learn on. Depending on your height, the seatpost usually comes pretty long, so you may have to cut it down with a hacksaw to get some flexibility on seat level.

As far as learning, I kept a little logbook of each time I went out. It honestly feels impossible at first (constantly falling in 360 degrees), but you just have to keep working on it. I did most of my learning at a school playground (lots of space to wobble around) and leaned against the school wall for support.