Flip, Grab, Lip or Special. To buy a trick, select the category you want. Click on a trick. If you have enough green to buy it, you will be asked to complete the transaction. Once you buy a trick, you can't get rid of it.
Special tricks cost the most of all of the available moves.
Of course, since this is due to their high scoring potential, it all seems fair in the end. You can only have five tricks assigned to buttons at once. This is to keep scoring from getting out of control. Skate Shop Head to the skate shop to buy new decks. The order you buy decks in is unimportant.