To do a wheelie on a bicycle, lean back and pull the handlebars upward while riding. On a motorcycle, the simplest way is to ride along while sitting toward the back of the bike, and increase your speed sharply so that the front wheel comes up.
Another way to wheelie on a motorcycle is to pull the clutch in, rev the throttle, then let the clutch out so that it slips. This will raise the front wheel more suddenly. It is important to start small and ride slowly when learning how to do a wheelie. The main danger is falling off the back when the front wheel raises too high.
If you feel the bike is becoming too vertical too quickly, use the rear brake to lower the front of the bike. Remember to only perform a wheelie where it is safe and legal to do so.