Some titles are free, some need a subscription, and some can be rented or bought on their own. Each title says which it is before you start.
Cancelling
Your account page has the subscription and a cancel option. Cancelling stops the next payment; you keep access until the period you have already paid for ends.
I have a code
Use Redeem a code in the account menu.
A payment went wrong
Check your account page first — it shows what has actually been charged. If that does not match what you expect, contact the site owner with the date and amount.