Know your current GPA. You can get this from the school or calculate it yourself. This is calculating the number of hours you have taken into the number of grade points you have accumulated. An "A" is worth four points, a "B" is worth three and so forth.
Decide how much you wish to improve your GPA. It is important to be realistic about your chances. If you only have one semester of school left, you are unlikely to change an overall 2.0 to a 3.5, however, if you are in your second semester, then a big change is possible.
Re-take classes in which you did poorly the first time. If you re-take a class, then the new grade takes the place of the old one. Changing an old "D" into an "A" can make a lot of difference. This option may not be available in high school, but it typically is in college. Re-taking a course in college will not have a negative effect, except that you will be expected to pay for the class a second time.
Learn new study habits. For example, learning to take good notes will help you retain information better. You can also use flash cards and create your own study guides.
Manage your time so that you prepare assignments and study for tests ahead of time. This will relieve stress and help you do better on those assignments.