How to install new outdoor motion sensing light
You need to locate a junction box somewhere in the attic that has a hot wire (not a switch leg) and a neutral in it. Look close to an existing light fixture in a room close to where you want the exterior light(s). From there you can feed the motion-activated lights with NM cable (Romex) from the attic. 

You can either cut boxes into the siding or mount surface boxes on the outside of the house. Whatever you feel comfortable with.

If the terminology is a bit confusing you should buy a book on basic wiring such as this one:
http://www.amazon.com/Step-Guide-Book-Home-Wiring/dp/0961920106/ref=sr_1_1?ie=UTF8&amp;qid=1450304277&amp;sr=8-1&amp;keywords=Home+wiring+step+by+step

Not an endorsement just a suggestion.