For online registration, one has to use a credit card and visit the link - http://www.ets.org/bin/getprogram.cgi?test=gre
If one has to register through this medium then he/she must have a credit card, which can be one of the following:
The phone numbers for registration are : 1-443-751-4820 or 1-800-GRE-CALL (1-800-473-2255)
Download Authorization Voucher Request Form. Fill it and mail to ETS.