Never done, myself. More than exposure, what you would get would be experience performing. And a little pocket change, if you're lucky. A busker has about five seconds to attract an audience before they walk on by. The ability to draw the listener in from the first note is what distinguishes really good performers from the rest.
I can't imagine it would be too hard. Find a place that has a lot of foot traffic and is safe. Make sure you know the local laws. If you see somebody doing it, stop and ask them about it. Then give it a shot. You don't need a larger repertoire since nobody will listen for more than a few minutes, if that. Obviously, get your parents' approval if you're underage.