An ollie is the most essential trick in skateboarding, and involves jumping with the board while you're riding it. To do an ollie, follow these steps:
Place your back foot on the tail of the skateboard, with the ball of your foot on the edge of the tail and your heel hanging off. Your front foot should be on the deck of the skateboard near the front trucks.
Bend your knees slightly and lean your upper body forward. This will help you gain more power and control when you ollie.
Quickly push down on the tail of the skateboard with your back foot while simultaneously pulling up on the front of the skateboard with your front foot.
As you push down and pull up, use your front foot to pop the nose of the skateboard up off the ground.
As the skateboard lifts off the ground, quickly bring your feet up and back towards your body, using your back foot to push the skateboard down again as you land.
It may take some practice to get the timing and technique right, but with a little bit of practice, you'll be able to do an ollie in no time.