[@Hippy]

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication2
{

    class Person
    {
        // Поля
        public string firstName;
        public string lastName;
        // Метод
        public void ShowFullName()
        {
            Console.WriteLine("Name is " + firstName + " " + lastName);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Person Petr;
            Petr = new Person();
            Petr.firstName = "Petr";
            Petr.lastName = "Ivanov";
            Petr.ShowFullName();
        }
    }
}

так все робит, чего у тебя не так?