Outdoor motion sensor lighting?

We have a regular outdoor lamp with a regular lightbulb that I would like to convert to a motion sensor light. It's on our side porch just outside the door and we don't leave the light on all the time. We just like to illuminate the area if someone is coming over at nighttime or when we are getting home after being out all day. Is it easy to convert this regular light to a motion sensor light? What type of hardware do we need to make this conversion? Is it just a lightbulb or is there a whole kit that needs embly? Is this an easy task or is it pretty involved? Best answer will go to the response that is detailed and easy to understand. Thanks!