regular Classes are out of 4 points, AP/IB/Honors will give u 5 for A
A-4 points
B-3
C-2
D-1
F-0
add one to your points if its an AP class, so A=5, B=4 etc.
GPA is the average of all the points you got in your classes, so a 4.0 would mean you have all As in regular classes or have Bs in some APs, which is pretty good.