help needed

Hi all, I have to write a small schoolproject, a simple calc.Here is my code:
using System;
class Simple_Calculator
{
    public static void Main()
    {
    double a,b;
    string c;
    double ut;
    char ag = ' ';

Console.WriteLine(" Tetta a ad verda eins og mjog einfold reiknivel ");
  Console.Write(" Write number: ");
  a = Convert.ToDouble(Console.ReadLine());
  Console.Write(" Choose op ( +,-,*,/,) ");
  c = Convert.ToString(Console.ReadLine());
  Console.Write(" Write number: ");
  b = Convert.ToDouble(Console.ReadLine());


  if ( ag == '+')
  Console.WriteLine( ut = a + b );
  else if ( ag == '-')
  Console.WriteLine(    );
  else if ( ag == '*')
  Console.WriteLine(    );
  else if ( ag == '/')
  Console.WriteLine(      );
  else
  Console.WriteLine("You have to write a number ");

Console.ReadLine();
}}
I cant get no output and I think it can be more simple when you write the numbers.
If someone can help me get this to work would be super
thanks
Geiri