firstly you add the namspace....
using System.Data.SqlClient;


private void button1_Click(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection();
            con.ConnectionString = "Data Source=SR; Initial catalog=nadeem;  user id=sa; password=<123>";
            con.Open();
            SqlCommand cmd = new SqlCommand("insert into login values ('" + textBox1.Text + "',' " + textBox2.Text + " ')", con);
            SqlDataReader dr = cmd.ExecuteReader();
            MessageBox.Show("save your data");
            con.Close();
        }


Regards-Nadeem.