Press Play on the right side of the screen, followed by Play in the middle of the bottom of the screen. While the dirt bike you ride in this game may be small, that does not mean it is slow. On the contrary, you can use the up arrow key to immediately start speeding through the mountainous terrain.
As you do, you will want to come into contact with the star bubbles. Not only will they increase your already fast speed, but they will also add to your score. In addition to the up arrow key, you will also use the left and right keys for balance. Just keep in mind that since this is a small bike, it only takes a small tap of either key to move the balance of the bike in that direction.
ARROW KEY UP: Gas
ARROW KEY DOWN: Brake
ARROW KEY LEFT and RIGHT: Level