You need to arrange Live Neutral and Earth to be routed to your outside light. It may also need a switch.
You can not usually wire it from an existing switch - most light switches have live going in and live coming out and do not have a neutral connection.
You will need a hole in the wall to pass the cable through (long drill bit) and can pick up the spur (cable run to the light) from the most convenient place consistent with placing a switch or fused switch somewhere.
Lots of talk about it here...
http://www.screwfix.com/talk/thread.jspa?threadID=78575
Or you can engage a professional sparky...
Peccavi
April 2009