If you're playing games off WHDload, then you need TUDE or Degrader off aminet.net.
If you're just fooling around with most Workbench programs, then the early startup menu (hold down both mouse buttons) in Kickstart 3.1 should do the trick.
If you're playing games off original floppies, then NTSC/PAL switching will be done automatically and all you might have to do is adjust your monitor's vertical width to see the entire picture or to stretch it out.
