If you have a credit card that you can use for paying the GRE test fee, the easiest way to register for the GRE is online - either at the official GRE website or at the Thomson Prometric website, or even by calling the test center where you want to register. However, if you do not have a credit card, you can use a certified check or money order to pay but you then have to mail that to the Regional Registration Center where you plan to take the test.