login page asp.net sql

I have this code and need to complete it..

string conn_str = 
     @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\mydb.mdf;
       Integrated Security=True;User Instance=True";
SqlConnection conn = new SqlConnection(conn_str);

SqlCommand cmd = new SqlCommand("SELECT Password FROM Users WHERE UserName=@un", conn);
cmd.Parameters.Add("@un",  SqlDbType.NVarChar);

cmd.Parameters["@un"].Value = **???**;

conn.Open();
string pwd = (string)cmd.ExecuteScalar();
conn.Close();

I have some values in sql data: Tables: Users Username Password

Now in login page i have textboxNAME and textboxPassword and if user type right login info(that in database) it refers him to default.aspx

Try

cmd.Parameters["@un"].Value = textboxName.Text;

and

if(textboxPassword.Text.Equals(pwd))
{
    Request.Redirect("default.aspx");
}
else
{
//login failed
}