Hello, I want to make a C++ program that has kinda like a database of usernames and passwords. that data corresponding to that user will be shown. please explain in as much detail as possible.
You may start analyzing the stl's map (http://www.cplusplus.com/reference/stl/map/), then continue to study some encryption (DES?, AES? ...just for reference) and other security solutions.
But you should now that these kind of things are not for beginners.