You have declared
as a non-static method, but you are trying to call it as if it is static. So change your code in one of the following ways:
public static int returnNum (int x)
Program prog = new Program();
int xReturnTest = prog.returnNum(x);
But what you should really do is get a good tutorial on C# and learn it in a structured fashion. .NET Book Zero by Charles Petzold
] is an excellent starter.