Yeah I don't even know how to Calculate GPA...
Σ
i(
ci *
gi)/Σ
i(
ci), where Σ denotes "the sum of",
c is the number of credits for the class and
g is the grade earned in the class, for each class
i. Your school probably has a listing of weights assigned per letter grade, but the general standard (afaik) is:
A - 4.0
B - 3.0
C - 2.0
D - 1.0
Pardon the abbreviated summation notation, but hopefully it should suffice.

Say you take 3 classes, BIO101, ENG101, MAT110. Your report looks like this:
BIO101 - 4 credits, B grade
ENG101 - 3 credits, A grade
MAT110 - 3 credits, A grade
Then your GPA is [(4 * 3) + (3 * 4) + (3 * 4)] / (4 + 3 + 3) = 3.6.