The GRE is a computerized test, so there are no set test dates. You can take the test throughout the year and you must book an appointment at a testing center local to you.
The most convenient way to register for the GRE is online at www.gre.org. This is the website of the Educational Testing Service (ETS), which is the administrator of the GRE.
The cost to take the exam in the US is currently US$140. The fee to take the exam outside the US is US$170. ETS takes most major credit cards.
Please note that you can only take the GRE once per calendar and no more than 5 times within any 12-month period.