The membership is monthly auto-billing. This means you don't need to do anything after you've purchased it! For example, if you purchase your membership on 2nd of September, the first payment is taken that day. The following payment will be automatically charged on the same bank card you used for the initial purchase, on 2nd of October. You will always get an email receipt when your monthly payment has been made. You can find the bill date on these emails, or just check your bank statement.
If for example you don't have sufficient funds on your band account connected to the bank card you used, you will get an email telling you the payment was unsuccessful and your membership has been therefore cancelled. If this happens, don't worry, you can just buy the membership again.