Can I sign up for just one month?

Yes, you can sign up for one month.  When you sign up for a particular membership you can cancel it at anytime by going to your account settings page and clicking on the recurring fees tab.  When you click the "cancel" link your account will not be charged on the next billing date.  Your account will remain active until the expiration date but your account will not be active after the expiration date.  Your account will not be deleted, therefore if you choose to sign up at a later date then you will still have access to all of your practices, drills, drill lists, systems, etc...