The Credit hours for a course means the number of hours the course is been taught in a week. For a example if Math is been taught 3 hours in a week then the credit hours is 3.

Depending on the curriculum your school is using, assigning grades to scores varies from one curriculum to another. Using UK curriculum:
70 - 100 = A A is assigned 5 points
60 - 69 = B B is assigned 4 points
50 0 59 = C C is assigned 3 points
45 - 49 = D D is assigned 2 points
40 - 44 = E E is assigned 1 point
0 - 39 = F F is 0
GP = credit unit * points
Example if the credit unit for maths is 3 and you have an A, which means the point your pint is 5
then the GP for maths = 3 * 5 = 15

you have to calculate your GP for all the courses you took.

Then the GPA = total GP / total credit unit