Saving Line or file creation... or etc [in need of great help]

Code:

#include <stdio.h>
#include <string.h>
#include <conio.h>
#include <stdlib.h>
main()
{
      char username[10];
      char pword[20];
      char cpass[20];
      int choice;
      int looper=0;
      char ynchoice[]="yes";
      char ynanswer[20];
      
      
      do{
      
      system("cls");   
      
      printf("\n[1] Display forum users\n[2] Create new user\n[3] Check username and password\n[4] Exit\n");
      printf("Enter choice: ");      
      scanf("%d",&choice);
      switch(choice)
      
      {
                     case 1:
                          printf("1: Username\n  Password");
                          getch();
                          break;
                          
                     case 2:    
                          do{                      
                          printf("\nEnter new username: ");
                          fflush(stdin);
                          gets(username);
                          printf("Enter password: ");
                          fflush(stdin);
                          gets(pword);
                          printf("Retype password: ");
                          fflush(stdin);
                          gets(cpass);                  
                          if(strcmp(pword,cpass)==0)
                          {
                             printf("Save data? Yes or no: ");
                             fflush(stdin);
                             gets(ynanswer);
                             printf("Enter new user? Yes or no: ");
                             fflush(stdin);
                             gets(ynanswer);                                                     
                                                    }
                             else if(strcmp(pword,cpass)!=0)
                             {
                                 printf("Password is not match\n");
                                 printf("Input new user? Yes or no? ");
                                 fflush(stdin);
                                 gets(ynanswer);                           
                             }
                             }
                          while(strcmp(ynchoice,ynanswer)==0);
                                 
                          break;
                     case 3:
                          printf("Enter username: ");
                          printf("\nEnter password: ");
                          getch();
                          break;
                     case 4:
                          return 0;
                          break;
                     default:
                             printf("Invalid choice\nTry again");
                             getch();
                             break;
      
      }
      }
      while(looper==0);

      getch();
      
      
      
      }

Ok time for another project.
If you scan my code, you will know what I'm getting at.

I need some help about adding and saving some lines in a separate function (like I'm going to add a function that will serve as a database for the saved user names and passwords. Although my professor already told me that saving a line in a function through input method from the user isn't possible.).
If not possible, how about some file creation and how do I call and scan the file or something.

Many thanks for the future help.

Out of topic: Whew! I have 5 major projects... beats me

EDIT: Forgot to include that I need this in C language and not c++. I did it in C++ compiler because the c compiler won't let copy and paste.
So please help in C language library and syntax and functions. THanks.