Very easy -- scale the footage to fit the comp. Ctl-alt-f (win) or cmd-opt-f (mac) You simply have to choose a resolution. Personally, I'd go with 1280x720.
When ever you scale layers to fit the frame there is a danger of distortion. I always use Fit To Comp Height (Ctrl/Cmnd + Alt Option + Shift + H) or Fit To Comp Width (Ctrl/Cmnd + Alt Option + Shift + G). If you use Fit make sure you double check scale to avoid distortion. X and Y scale should be equal.