Does anyone know how to start this off?

Write a program to compute the grade for every student of a class. In addition, your program needs to output a summary report showing the total number of students, and the number of students receiving each letter grade. To indicate that all students have already been processed, use a number larger than 100 for the first assignment score of the student as the sentinel.
In other words, if the score for assign. #1 of a student is larger than 100; your program should report the collected summary and then terminate.

Yes, this gets asked on pretty much a monthly basis, so search the board.

But I would suggest you start with working out the grade for just one student to begin with.

I'd say start with a flowchart, then implement the case for one student, then two, and then N students.