Which language are you learning, C or C++? (The two languages are very different) and how far have you managed to get with the problem? you're likely to get more useful feedback if you post your attempt at a solution so far.
One way to begin with a task like this is break everything down is to bite-sized pieces. Often it helps to write some kind of informal pseudocode. (If not in a text editor, then on a sheet of paper)
certain parts of the problem should be very straightforward, such as converting a number of marks out of 50 into a percentage.
Other parts of the problem, such as determining the pass/fail status of a student will need a bit more thought - and, depending how much of the language you know, there are different ways you could solve it (You probably want to break the solution down into several functions, to make life easier, and avoid repeating code).