How do I make a coding for visiting a farm and need to put date bookinf

need to do a coding about online registration to visit a farm. In the coding, I need to put about name and password for the registered user and make like if else for new user. I also need to put choose a date to visit at the end of the coding. In the coding, I must include array, pointer, loop, file and everything in C programming. I hope somebody can help me since I am really really struggling with programming. I really hope someone helps me. Thank you

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

struct login
{
    char fname[30];
    char lname[30];
    char username[30];
    char password[20];
};

void login (void);
void registration (void);

int main (void)
{
    int option;

    printf("Press '1' to Register\nPress '2' to Login\n\n");
    scanf("%d",&option);

    getchar();

    if(option == 1)
        {
            system("CLS");
            registration();
        }

    else if(option == 2)
        {
            system("CLS");
            login();
        }
}

void login (void)
{
    char username[30],password[20];
    FILE *log;

    log = fopen("login.txt","r");
    if (log == NULL)
    {
        fputs("Error at opening File!", stderr);
        exit(1);
    }