Turn in ALL of your homework and assignments. Ask your teachers for extra credit, and most schools offer after school tutoring as well. It really isn't very difficult if you are determined to get good grades :)
Usually one school year is two semesters. So if you got a 3.0 in the first semester, you can improve that by focusing more in your classes by the second semester.You can set goals in order to get a better grade like doing all your homework, and studying for exams. For example, my GPA went up to a 3.95 from a 3.91 in the first semester because I decided I didn't want any B's.