Is that all the instructions you were given? Have you ever seen a telephone or cell bill? Among other things it will contain a detailed list of all the chargeable phone calls you made, how long the calls were, the cost per minute, and the total cost for each call. Your program will most likely have to contain similar information. so, you can create a class that allows you to enter the info for a single call and save it to a file. At some time later your program will prepare a bill.
Now, think about what your program needs to do, and create a menu that allows you to do them.