Use a brake, lower the gearing, and learn to skid and skip properly if you're going to do that sort of thing. And yes, you are messing up your knees, probably not permanently but the world is full of people like me who did a lot of dumb stuff to their knees at 16 and now 10 years later have to be completely psychotic about fit and form to ride without knee pain.

Go to the shop you bought it from, if they're not jackasses they'll fix the gearing or put on a brake and let you pay it off. Of course, anyone who sells a bike geared 48x15 in a hilly area and doesn't offer to fix the gearing or put on a brake is a jackass anyway, so who knows.