[[nightynvid]]

suponiendo que la clase que quieres crear se llama MiFormulario

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            MiFormulario frmNuevo = new MiFormulario();
            frmNuevo.Showdialog();
        }
    }
}