Practice riding on a lawn (NOT on asphalt). That way, when you fall (and you will, controlled, it's fine) you'll only end up with grass stains, not real scrapes.

And DON'T use training wheels--they will only slow your learning process down.

You can look at this site for tips, and if you live in the Boston area, you can go there for lessons: http://www.bicycleridingschool.org/

oh, and wear a helmet.