Send in this form 8050-88A filled out and notorized with your 3part from 8050-1 and $5 to:

FAA Aircraft Registry, Mike Monroney Aeronautical Center, PO Box 25504, Oklahoma City, OK. 73125

Several weeks later you should get your registration card back in the mail.  It is thicker cardstock about the size of a postcard.  This card must be kept on your aircraft along with the Airworthiness Certification.  **the registration card must be identical to the data plate.  So have it handy when you order your data plate and double check for accuracy.