C# Registration problem

protected void btnLogin_Click(object sender, EventArgs e)
   {
       SqlConnection cnn = new SqlConnection("Add your connection String here.");
       SqlDataAdapter da = new SqlDataAdapter("Select * from tblRegistration where username= '" + txtusername.Text + "' and password = '" + txtpassword.Text + "'",cnn);
       DataSet ds = new DataSet();
       da.Fill(ds);

       if (ds.Tables[0].Rows.Count != 0)
       {
           Session["username"] = ds.Tables[0].Rows[0][0].ToString();
           Response.Redirect("Home.aspx");
       }
   }