As is customary with all of Amazon’s subscription services, there’s a 30-day free trial on offer when you first sign up. If you aren’t happy with the Prime service, you’ll need to manually cancel your membership before the 30-day trial ends or you will be charged the full monthly fee the following month.
You can, of course, sign back on any time again, but you won't get that 30-day free trial if you've already participated previously.