While travelling at a fairly slow pace grab your front brakes hard. Let your body lift upwards and towards the front of the bike. Put most of your weight on your hands on the handlebars and, without removing your feet from the pedals, unweight your feet. To gain more height get as far behind the seat as possible and push the bars away, at the same time let the bike pivot underneath you. As soon as you feel you are going to fall forward lean back or let off the brakes. 

Once you have the endo figured out and can do it for a larger period of time, you're ready to learn a nose wheelie. It is a little more challenging than an endo. Instead of completely stopping on the front wheel you coast on the front wheel. Perform the same procedures as an endo, but as you reach your balance point let the brake go and coast. Gently apply the front brake as needed to compensate for front and rear balance. A nose wheelie is easiest on a gradual slope, as gravity does all the work, so no speed is required.