Registering for GRE
To take the GRE, you'll have to create an account on the GRE website and sign up for a test date - this is also how you'll get your scores and send them to schools. Because the GRE is taken on a computer (CBT), GRE test dates are actually very flexible, and you have a lot of space to pick a date that suits you. You'll pick a test center near you and take your pick from a list of times available - which specific times are open depends on your test center.
GRE Content
The GRE is about 3 hours and 45 minutes long, including a 10-minute break. It tests you in three subject areas:
Analytical Writing
You'll only get one Analytical Writing section on the test, and it will always be first. You'll have to write two essays in this section.
Verbal Reasoning
it's mostly vocabulary and reading comprehension. You'll get two Verbal Reasoning sections on the test.
Quantitative Reasoning or math
You'll get two Quantitative Reasoning sections on the test.
Everyone who wants to take GRE has to take all the sections regardless of what you're going to grad school for. But here's the actual picture of that: you aren't expected to actually be an expert in any of those fields.
Except the vocabulary, the GRE is primarily a test of problem-solving and reasoning skills, not subject-area knowledge or memorized facts. So, the math questions are hard because they're logically difficult to think through, not because they test really high-level concepts of mathematics. If you're going to grad school in the social sciences and haven't taken math since your general education requirements, don't be panic about it - the concepts are the same ones you learned in your high school studies, and you won't have to do any fancy theoretical calculus or analytical geometry.
The same is true for the reading sections. You won't have to apply specialized literary terms or know a lot of background critical theory. So, if you're an engineer or a math person, don't panic and start cramming Proust and Keats. It's all about understanding what you see on the page, not knowing special field-specific jargon.