The first step in ECFMG registration is by requesting a USMLE ID which is by going to this web page
https://secure2.ecfmg.org/requestinstructions.asp
For more detailed instructions about how to register read this thread
http://www.usmle-forums.com/ecfmg-forum/362-ecfmg-registration-steps.html
If you are planning to take CK before Step 1 then I invite you to read this thread
http://www.usmle-forums.com/general-topics-recommended-threads/1223-step-take-first.html
https://secure2.ecfmg.org/requestinstructions.asp
For more detailed instructions about how to register read this thread
http://www.usmle-forums.com/ecfmg-forum/362-ecfmg-registration-steps.html
If you are planning to take CK before Step 1 then I invite you to read this thread
http://www.usmle-forums.com/general-topics-recommended-threads/1223-step-take-first.html