Take the weighted average:
[Sum of ] (Credits * Grade Point) / (Total Credits Attempted)
Where sum is the total you add up for every credit*grade point from every single class you took in your college career with some exceptions (see below)
Where Grade points are A=4, B=3, etc. On a +/- system you need to adjust such that A- = 3.7, B+ = 3.3, etc.
In total credits attempted you would not include "W" grades since those had no weight in the formula; Do include F's however (hopefully you got none of those!)
So let's say you took only 2 courses in your life, both being 3 credits and you earned an A in one and a D in the other:
[(3*4) + (3*1)] / 6 = 2.5
Also, you wouldn't include any remedial-type courses that you may have taken at a CC. Common examples are introductory algebra classes or basic writing courses due to a low placement test score.
It's easiest to do this in a spreadsheet so you can keep track of the numbers easily and not make any errors nor have to do the actual calculations more than once -- it's easy to screw up somewhere on a calculator with so many entries.