The first thing that you'll want to do is get ahold of your grades. If you know them off the top of your head, that's okay too.
The next thing you need to know is how the grade point system works. For every A you have, you'll assign 4 points. A B is 3 points. A C is 2 points, and a D is 1 point. For every F, you'll get 0 points.
Now, let's say I had a B, A, C, and B. My numbers will look like this, 3, 4, 2, 3.
Let's take that step above, and figure out the GPA. You'll want to total up your numbers, and divide by the number of classes you took. Up above, we will have a total of 12, and 4 classes, giving us a 3. Our GPA is a 3.