You can easily create a prepaid membership by charging the member upfront and choosing the membership duration (e.g. 3 months, 6 months, 1 year, etc.).
When you Add a Membership Plan you have the option of adding a Billing Period and an Optional Term Length. If you're setting up a one-time membership purchase (pre-paid, upfront) then you're going to want to fill out both of these fields.
Billing Period: How frequently does this membership get billed
Optional Term Length: Setting a term length allows you to have a plan cancel or auto-renew after a set period of time
*However, if you are trying to create a one-time payment for just one month, we suggest setting it up as an attendance pack and choosing the number of member attendances (e.g. 30) and set it to expire after one month.