Method that returns greater value of two numbers

You can use the built in Math.Max Method

Since you are returning greater number, as both are same, you can return any number

public static int GetMax(int first, int second)
{
    if (first > second)
    {
        return first;
    }

    else if (first < second)
    {
        return second;
    }
    else
    {
        return second;
    }
}

You can further simplify it to

public static int GetMax(int first, int second)
{
  return first >second ? first : second; // It will take care of all the 3 scenarios
}

static void Main(string[] args)
{
    Console.Write("First Number = ");
    int first = int.Parse(Console.ReadLine());

    Console.Write("Second Number = ");
    int second = int.Parse(Console.ReadLine());

    Console.WriteLine("Greatest of two: " + GetMax(first, second));
}

public static int GetMax(int first, int second)
{
    if (first > second)
    {
        return first;
    }

    else if (first < second)
    {
        return second;
    }
    else
    {
        throw new Exception("Oh no! Don't do that! Don't do that!!!");
    }
}

but really I would simply do:

public static int GetMax(int first, int second)
{
    return first > second ? first : second;
}