To end your Amazon Prime trial and return your account to a standard free one, find the Your Account page on the Amazon website and then scroll down to Your Memberships & Subscriptions near the bottom.
Select this, and you'll see your Prime membership (as well as any other subscriptions you have linked to the account. Press the button for Prime membership settings, then press Manage membership, then press Manage.
You'll here have options for pausing or ending your membership. You can also find here an option to set a reminder before your membership needs renewing, so you don't forget and accidentally roll into a paid plan.