Start at the bottom of a halfpipe. Give yourself a push to get going. When riding on a side of a halfpipe, bend your knees.

When you feel you're loosing speed, stretch your knees. Push your board high on the ramp.

Then put all of your weight back on the board. Push it down the ramp with lots of power. Try to push the board trough the ramp.

Now you should be riding to the other side of the halfpipe with more speed than you had when you started.

Repeat this progress on the other side. If you keep doing this, you will be going higher and higher.
In the beginning, it may not work, but just keep practising!