Oooh. You didn't use the search. If you have a school nearby, riding on a volleyball court, from post to post is one idea. You could also try the tetherball court.
I've never ridden one, but that's the advice I heard 1,000 times when I worked at a shop with a hardcore uni rider.
The best place to learn is on a very steep hill, preferably wet grass but icy pavement will also work well. Once you get the hang of it there you can ride it anywhere.
What about some kind of harness system that could hold you up if you fall off it then you have a rope tied to the seat and while your hanging there u can pull it back under you and keep riding