You'll be billed once a month, starting 30 days after you sign up. If you choose to cancel your membership by the end of the first month, you will not be billed.
The same applies during the following months. If you choose to cancel your membership in the middle of a given month, you will not be billed at the end of that month.
Billing takes place on the same date each month, for as long as you are a Playster member.