Caesar cipher

[hitman_38]

Hi everyone
I have been trying to create a simple encryption program it will read the txt from a .txt and then if plaintext encrypt it or if encrypted decrypt it to plaintext using the caesar's cipher I just can't get my head around as I am only new to c++ programming

Any help or ideas would be great

Here's what I have so far:

#include <iostream>//for input and output
#include <fstream>// for working with files

#define isupper(a) ((a)>= 'A' && (a)<= 'Z') 
#define islower(a) ((a)>= 'a' && (a)<= 'z') 

using namespace std;
ifstream inFile;
ofstream outFile;
char PlainText[100], CipherText[100] ;
int main() 
{
    {
       outFile.open("test.txt");
       (!outFile.eof() << PlainText[100]);
       for ( int i = 0 ; PlainText[i] ; i++ )
       {
           if ( isupper(PlainText[i]) )
            CipherText[i] = 25 - (PlainText[i] - 'A') + 'A' ;
           else if ( islower(PlainText[i]) )
            CipherText[i] = 25 - (PlainText[i] - 'a') + 'a' ;
           else CipherText[i] = PlainText[i] ;
        }
       outFile << CipherText << endl;
    }
inFile.close();
system("pause");
return 0;
}

Thanks in advance