My parents threaten me with my future?

It's always something dumb like I didn't throw something away or I forgot something somewhere and they totaly flip out and say something like they'll take away my computer so I can't do my homework, they won't drive me to school, they won't let me work anymore. If I can't get good grades or work then I have no future whatsoever and sometimes they even do what they are going to say. They are going to actually destroy my life for their own egos! Why do they do that? What can I do?