Any owner of a small UAS who has previously operated an UAS prior to December 21, 2015, must register no later than February 19, 2016.
 
Owners of any other UAS purchased for use as a model aircraft after December 21, 2015 must register before the first flight outdoors

Registrants will need to provide their name, home address and e-mail address. Upon completion of the registration process, the web application will generate a Certificate of Aircraft Registration/Proof of Ownership that will include a unique identification number for the UAS owner, which must be marked on the aircraft.

Owners using the model aircraft for hobby or recreation will only have to register once and may use the same identification number for all of their model UAS. The registration is valid for three years.

The normal registration fee is $5, but the FAA is waiving this fee for the first 30 days (from Dec. 21, 2015 to Jan 20, 2016).

To view the full article:
http://www.faa.gov/news/press_releases/news_story.cfm?newsId=19856&cid=TW378