Do you have a job? Go push a mop for a couple years. It will motivate you to do well in school and has the added benefit of giving you some money for university.
I actually second this. I worked a bunch of odd jobs starting at 15 years old (you were supposed to be 16, so I had to get consent from the principal to flip burgers). As I was taking the garbage out while working at a grocery store, it just hit me: I can't do this for the rest of my life. I believe I was around 18 years old, so maybe a little older than the OP. I joined the local chamber of commerce and started consulting that summer and made more money than the previous 3 summers, and I enjoyed the work I was doing.
It's good to work awful jobs. It builds healthy character and eventually it'll probably hit you that you really want to be doing more intellectual things, which conveniently pays more money than most physical labor.
