How do you get motivated to study?

I'm in year 12 (final year of high school) and I need to finish in the top 7% in the state to get into the Commerce/Arts course I want to do, but I can't seem to do enough work. Whenever I sit down to study I'd much rather watch a DVD than study, and by the time I've watched 3 eps of big bang theory or whatever its only 1-2 hours before i gotta go to bed. Its only the start of the school year but I can see its starting to be a problem.