Code:
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int Menu(void);
void Encrypt(void);
void Decrypt(void);
string EncryptLine(string,int);
string DecryptLine(string,int);

int main(void)
{
        int c = Menu();
        while( c != 3 )
        {
                switch(c)
                {
                        case 1:
                                Encrypt();
                        case 2:
                                Decrypt();
                }

        }
}

int Menu(void)
{
        int c = 0;
        cout << "Press [1] to Encrypt" << endl'
        cout << "Press [2] to Decrypt" << endl;
        cin << c;
}

void Encrypt(void)
{
        string encryptedLine;
        string line;
                //Stuff to get a line and the key
        EncryptLine(line,key);
        //stuff to write out lines
}

void Decrypt(void)
{
        string decryptedLine;
        string line;
        // Stuff to get a line
        DecryptLine(line,key);
        // stuff to write out lines
}
this is the example i was given to go of off and i have no idea where to even start, we are also given 2 files, Number.dat, the only thing in that file is the number 3, and then we are given Unencrypted.txt, which contains 3 lines of text.

I am not asking for this to be done for me i am merely asking if anyone can give me any tips or hints on where to start, i am extreamly confused

Edit: We have to pull in the information from Unencrypted.txt, use the Number.dat as the key, and output it to a new file names Encrypted.txt