I was wondering how to setup a subscription system for my website, I will be accepting payments via paypal but, I was wondering how I would store this information in my mySQL

I know that the page the user is redirected to could have a function to add one month to the users account but one problem is how do I prevent the user from refreshing the page and repeating the command.

Another thing is how would I set it up so every month for example it adds a month of time and also removes a month of time. Will I have to setup a cronjob or is there an easier method? I haven't mastered PHP yet or client to server communication or database structure. I'm only 13 : ) .