Nollie: A nollie is basically an ollie, but you do it by pushing down on the front end of the board, instead of the back end. You should learn how to ollie before attempting this trick.
To nollie:
Put your front foot on the nose of the skateboard, and your back foot on the middle of the board.
Push down hard on the nose so that the back pops up. Slide your back foot toward the tail to get the front off the ground.
Use your back foot to level out your board in the air, the same way you would in an ollie, but by using your back foot instead of your front foot.