Formatting is pretty simple: you just provide a parameter to the ToString (or the parameter number in string.Format / stream.Write operations):
double d = 123.4567;
double z = 123.0000;
Console.WriteLine(d.ToString("0.00"));
Console.WriteLine(z.ToString("0.00"));
Console.WriteLine("{0:N2}", d);
Console.WriteLine("{0:N2}", z);
Will generate:
123.46
123.00
123.46
123.00