Find a grassy (slight) slope (park or something like that), lower the seat so you can put both feet on the ground while seated, and just coast down, with feet out. Take it nice and slow. When you get the hang of balancing, try getting your feet on the pedals while coasting. Then try gentle turns. When you're comfortable, move the seat up to proper height, then work on propelling yourself with the pedals.
You can be riding in a couple of hours.