Drive your dirt bike as best as you can and try not to let the obsctales defeat you. Control the dirt bike by using the left and right arrow keys and make sure you will not crush. If you do crush, you will have to start the level all over again.
Collect as many dollars as possible and use them wisely on the upgrade panel. You have two possible upgrades: speed and bike control. You will notice that some level can only be passed if your dirt bike is able to gain a nice speed level. Good luck!