Developing good study skills and study habits is vital to your success in high school! Read the following College Board pages for good info about various aspects of studying.
  If you think you may have trouble in one or more of your classes seek help immediately!
Use your teacher as a resource
If your teacher notices that you are taking the initiative to seek help on your own, s/he will respect you more, see that you value your education, and, most likely, s/he will be more willing to offer you additional help in a timeframe that works effectively for you.
S/he may be able to schedule extra times before or after school to help you out in addition to class time.
Your high school may also have a free peer tutoring service program, so look into that or ask your teacher!
Use your peers as a resource!
Find someone that seems like they deeply understand what is going on in your class.
Look for someone who asks intelligent questions or someone you know is doing really well but remember, the person getting the best grade in the class is not necessarily the person who really understands what’s going on the most.
Make sure to look for someone who also has good communication skills, someone who could explain the material well.
Ask him/her if s/he would be willing to help you outside of class.