SGPA stands for Semester Grade Point Average. It is a grading scheme used by many universities. Mathematically, it is the weighted mean of the grades achieved in various subjects with the credits alotted to the subjects acting as weights. To Calculate SGPA, you must know the credits alloted to each subject. The credits are usually mentioned on the syllabus.
Once you know that, multiply the credit alloted to each subject with the grade achieved in that subject and add the products. Divide this sum by sum of all credits to get your SGPA.