Gather your grades. You can accomplish this by pulling out your high school report cards. If you are in the middle of the semester, looking at the grades on your assignments will suffice.
Convert the letter grades to numbers. The scale for this is as follows: A= 4 points, B=3, C=2 and D=1. Failing or incomplete grades count as zero points. For every "plus" grade given, add 0.33 to the base grade, so a B+ would be worth 3.33 points. For every "minus" grade, subtract 0.33 from the base grade. A grade of B- would be worth 2.67 points.
Add the numbers together from each of your classes and divide by number of classes you took. This calculation will produce your GPA. For example, if you have two B grades (6 points) and three A grades (12 points), you have 18 total points. Divide that by five classes, and your GPA is 3.6. If in four classes you had an A- (3.67 points), a B (3 points), a B+ (3.33 points) and a C+ (2.33 points), your grades would total 12.33 points. Divide by four classes, and your GPA is 3.08.