Go to Home Depot or another hardware store and buy a sensor for about $12. Also buy an electric gang box with wings for $1.50. Twenty-five feet of 3 strand wire will do it. That would be 12-3 gage wire.
The switch to your exterior light beside the front door should be located in the interior side directly below the exterior light. Remove the exterior light to reveal the wiring. Next, you need to cut a rectangular hole on the exterior side the size of the gang box insert. Now install the gang box making sure it is secure. Fish your wire through from the gang box to the light box and wire it according to the instructions included with the sensor. Re-install your light. Finally, test the sensor by putting black tape over the sensor eye. Turn on the light switch. Wait a couple of minutes. If the light comes on then your sensor works.
Now all you have to do is to remember to leave your light switch on at all times. Let the sensor do the rest.