I have created a simple login page but I got problem with getting logged in user ID

public partial class LoginwithEncryption : System.Web.UI.Page
{
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["conStr"].ConnectionString);
        con.Open();
        SqlCommand cmd = new SqlCommand(
            "select * from dbo.UserInfo where Login =@Login and Password=@Password", con);
        cmd.Parameters.AddWithValue("@Login", txtUserName.Text);
        cmd.Parameters.AddWithValue("@Password", txtPWD.Text);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        da.Fill(dt);
        if (dt.Rows.Count > 0)
        {
            Response.Redirect("StartPage.aspx");
        }
        else
        {
            ClientScript.RegisterStartupScript(Page.GetType(), "validation",
                "<script language='javascript'>alert('Invalid UserName and Password')</script>");
        }
    }
}

How can I get ID of user after login (I'm able to login) ? I tried few methods but it won't work?