void display(); // Function prototyping
display(); // You spelled diplay. You don't need to type void. You make a new function if you do that.
// What you did is called function overloading. Giving two or more functions the same name.
// So if you want to call a function in main only use the function it's name!
// if you want you can use system("CLS"); here. You must include windows.h for that
cout << "Ok" << endl;
cout << "This is my first function program!" << endl;