How to calculate GPA in 4 steps: grade points, credit hours, quality points, divide. Worked example, the 4.0 scale and a calculator that shows its working.
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Calculus, B, 4 credits: 3.0 x 4 = 12.0 quality points. English, A-, 3 credits: 3.7 x 3 = 11.1. Psychology, A, 3 credits: 4.0 x 3 = 12.0. Chemistry lab, C+, 1 credit: 2.3 x 1 = 2.3.
Totals: 37.4 quality points over 11 credits. GPA = 37.4 / 11 = 3.40.
The common mistake is to average the grades without credits: (3.0 + 3.7 + 4.0 + 2.3) / 4 = 3.25. That treats the 1-credit lab like the 4-credit calculus course, and it is wrong whenever credits differ.
GPA = total quality points / total credit hours, where a course's quality points are its grade points (A = 4.0, B = 3.0 and so on) times its credit hours.
The grade points for a course multiplied by its credit hours. A B in a 4-credit course is 3.0 x 4 = 12 quality points.
If every class carries the same credit, which is common in high school, just average the grade points: add them and divide by the number of classes.
Convert each class percentage to a letter grade using your school's scale, then use the letter's grade points. There is no single formula from an overall percentage to a GPA.
Report it the way your transcript does, usually to two or three decimals. On a resume, do not round up to the next tenth: a 3.47 is a 3.47, not a 3.5.