How to connect login forms to another forms? (I don't understand)

It was first time to join developer forums (CMIIW) to learning coding for my college. C++ Builder 6 was used in my department

So I had learning simple login forms, but I want to try to make "Login" button accessing another forms (or .cpp) such database system if login successful ("if" code) and I don't know what code for this. I had find this but I cannot figure it (because I am new student and I never know about app coding).

For example, login.cpp --> Press "login" --> "If-statement" --> "ShowMessage" --> database.cpp

What's code for accessing another form in "if-statement"?

Thanks :)

Sorry for my bad English