All good suggestions so far. I started a degree in Biology, but gave it up as far too boring. Physics was much more interesting. That isn't to say it was easy, but the reality of studying any subject is that if it is interesting you don't mind the fact that it is hard. I ended up with a masters in elementary particle physics before I ran out of money.
To emphasize the best advice given to date:
do not get behind! Learn equation and definitions as you go.
study Physics every day (except from 5:30 p.m. Friday until 5:30 p.m. Sunday)! Read, work problems, and memorize definitions
work in a study group of 3 or 4 that actually studies! Many minds working together will see solutions and explanations that one mind alone may miss. It is also good to have someone to vent frustrations to--get it out of your system then get back to work.
ask for help! If not from your teacher, then from another teacher (math teachers usually have had some Physics) or from college students who have had physics.
have fun! Physics really is fun and once you solve a hard problem all on your own and get the right answer you will experience a rush of satisfaction which is difficult to match.