Scratch

If you want part of your ground to be poisonous:

when gf clicked
forever
  if <touching color [#FF0000]?>
  change [Lives v] by [-1]
  go to x: [0] y: [0]
end

This example kills the layer whenever they touch the color red (make this the color of the thing you want to hurt him. Mine continues on to change a life counter by 1 and reset him center-stage. However, past it sensing if he has touched the color you need, you can have whatever action you want triggered. (Most likely you would want to reset him to the start of the level, or give a game over screen)