Try this:

using System;

class Program
{
    static void Main()
    {
        while(true)
        {
           Console.Write("Enter Employee name or 'Quit' to quit : ");
           string name = Console.ReadLine();
           if (name == "Quit") return;
           double hoursWorked;
           double hourlyRate;

           while(true)
           {
             Console.Write("Enter Hours Worked : ");
             bool isValid = Double.TryParse(Console.ReadLine(), out hoursWorked);
             if (isValid) break;
             Console.WriteLine("Not a valid number. Please try again");
           }

           while(true)
           {
             Console.Write("Enter Hourly Rate of Pay : ");
             bool isValid = Double.TryParse(Console.ReadLine(), out hourlyRate);
             if (isValid) break;
             Console.WriteLine("Not a valid number. Please try again");
           }

           Console.WriteLine();
           double grossPay = CalculateGrossPay(hoursWorked, hourlyRate);
           Console.WriteLine("Gross pay   = {0:C}", grossPay);
           double stateTax = CalculateStateTax(grossPay);
           Console.WriteLine("State tax   = {0:C}", stateTax);
           double federalTax = CalculateFederalTax(grossPay);
           Console.WriteLine("Federal tax = {0:C}", federalTax);
           double FICATax = CalculateFICATax(grossPay);
           Console.WriteLine("FICA tax    = {0:C}", FICATax);
           double netPay = CalculateNetPay(grossPay, stateTax, federalTax, FICATax);
           Console.WriteLine("Net pay     = {0:C}", netPay);
           Console.WriteLine();
         }
     }

     static double CalculateGrossPay(double hoursWorked, double hourlyRate)
     {
         return Math.Round(hoursWorked * hourlyRate, 2);
     }

     static double CalculateStateTax(double grossPay)
     {
          return Math.Round(grossPay * 0.04, 2);
     }

     static double CalculateFederalTax(double grossPay)
     {
          return Math.Round(grossPay * 0.23, 2);
     }
 
     static double CalculateFICATax(double grossPay)
     {
          return Math.Round(grossPay * 0.14, 2);
     }

     static double CalculateNetPay(double grossPay, double stateTax, double federalTax, 

double FICAtax)
     {
          return Math.Round(grossPay - (stateTax + federalTax + FICAtax), 2);
     }
}