You want your website to work on phones and tablets, right? To be "responsive"? You may be thinking that you'd like to use a popular CMS like WordPress or Drupal, but you're not sure whether it'll be "responsive". Good news: any quality CMS can be used to build a responsive website, because "responsiveness" happens in the theme.
It's true that some CMSs can't be themed to be responsive, but their theming potential is so limited that you'd probably want to avoid them anyway. A good CMS can look like anything, and this certainly includes both WordPress and Drupal.
A responsive theme will often boast this fact (although responsiveness is fast becoming a requirement, not a feature, for new themes). But you can also use a free tool in Firefox to easily test whether a theme is responsive at different screen sizes.