scroll down to the bottom of this or any Amazon webpage
click on the word Help
select Need more help?
click on Contact us
the most common way most people sign up for Prime without realizing they do so is to click on the free shipping option which, if you read all the verbage that comes up afterwards, signs you up for a free trial of prime and explicitly states that you will be charged after 30 days if you do not cancel. Another way people sign up is clicking on any of the "read for free" or "watch for free" options as they will do the same
Click on "Help" at the bottom of this page, select "Need more help?" and then "Contact Us". Sign in. Select "Prime and More" and then in Section 2 select "Amazon Prime" and "Cancel Prime Membership". You'll then see the Phone option. Select that and Amazon will phone you in less than a minute. When you speak to them, ask how you became signed up or you risk being in the same situation in the future.