It varies by school and whether you're on a 5.0 or 4.0 scale, but I think the usual method is:
A=4.0
B=3.0
C=2.0
D=1.0
F=0.0
Again, it depends on the school...some have variations like
A+=4.0
A=3.75
A-=3.5
B+=3.25
B=3.0
B-=2.75
C+=2.5
C=2.25
C-=2.0
D+=1.75
D=1.5
D-=1.25
F=1
You just add them together and take the average.
So going by the first system, if I have 3 A's and a B, that's 3 4.0's and a 3.0 averaged together, which gives a 3.75 GPA.
Hope that helps.
-Forsaken