want to define roles and authentication on my login page

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ASP.NET

i have made a login page i want to thing in it 1st i want to define roles nad 2nd make my code authentication soo no one can access from url.
please help me for that

try
            {
                SqlConnection con = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=C:\Documents and Settings\Administrator.LENOVO-74FE9906\Desktop\Orignal Project Part 1\Inventory Management System\IMS.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True");
                string query = "SELECT [user].* FROM [user] where u_name='" + txtusername.Text + "' and pwd='" + txtpassword.Text + "'";
                SqlCommand com = new SqlCommand(query, con);
                con.Open();
                if (txtusername.Text == "admin")
                {

                    if (com.ExecuteReader().HasRows)
                    {


                        Response.Redirect("index.html");
                        Label3.Text = "successfully";



                    }
                    else
                    {
                        Label3.Text = "not successfully";
                        txtusername.Text = "";
                        txtpassword.Text = "";
                    }
                }
                else if (txtusername.Text == "user")
                {
                    if (com.ExecuteReader().HasRows)
                    {


                        Response.Redirect("userinventory.aspx");
                        Label3.Text = "successfully";



                    }
                    else
                    {
                        Label3.Text = "not successfully";
                        txtusername.Text = "";
                        txtpassword.Text = "";
                    }
                }

                con.Close();
            }
            catch (Exception ex)
            {
                Console.Write("" + ex.Message);
            }