Clear Screen: clrscr() with Borland, only way I found with VC++ was system("cls") or one of the Console functions. For Unix, look at curses.

File: I would suggest using the file streams with C++, they're a lot easier to use (and more stable):
Code:
#include <iostream>
#include <fstream>

using namespace std;

void somecode() {
    ifstream input("myfile");

    if(!input) {
        // something went wrong, bail out here
        return;
    }

    char key[64]; // or however long your key is
    input.read(key, 64);
}
If you don't know how long the key is, use a std::string object.

Prototype: It's only concerned by the types you use, in this case you pass two ints to it, which is fine (it only uses the names within the function -- in fact you can supply only the type to the prototype and the definition of the function, you just can't reference it like that).

Return values: The function return value can be used almost anywhere:
Code:
#include <iostream>

using namespace std;

int function(int, int);

int main() {
    int i = 5;
    int j = 4;

    cout << function(i, function(j, j)) << endl;
}

int function(int thing, int that) {
    return thing + that;
}