Retrieve Users_ID and place in session from formsauthentication

Hi i have the following which authenticates the user, how do i go about getting back the USERS_ID and placing it in a session, what i have tried so far does not work, this is my code;   string UserPassword = FormsAuthentication.HashPasswordForStoringInConfigFile
("P283NC9G" + txtPassword.Text, "SHA1");
SqlCommand command = new SqlCommand("User_login", conn);
command.Parameters.Add("@UserName", SqlDbType.VarChar).Value = txtUsername.Text;
command.Parameters.Add("@Password", SqlDbType.VarChar).Value = UserPassword;
command.Parameters.Add("@USERS_ID", SqlDbType.Int);
command.Parameters["@USERS_ID" ...