Login/register web service
When user gets saved in DB using user_save() it returns the user object. All you have to pass it to $GLOBAL['user']; and save session

   //Here is your previous code I assume where you have user_save()

   $user = user_save($userData);

   if($user) {
        drupal_save_session(FALSE);      
        $GLOBALS['user'] = $user;
        drupal_save_session(TRUE);
   }


Hope this may help you