Can I sign up for just one month?

Yes, you can sign up for one month. When you sign up you will want to choose the monthly option.

You can cancel it at anytime by clicking the "My Account" button in the upper right corner (while logged in) and then choosing "subscription".

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 be sure to use the same email address and you will still have access to all of your practices, drills, drill lists, systems, etc.