Do I have to pay every month or is it automatic?

I selected the monthly plan and i´d like to know... 

It should be automatic as long as you don't cancel your active subscription then there would be a recurring payment.

I have also checked that the payment gateway that was used was FastSpring, automatic charge should not be a problem. If you are going to cancel your subscription just go to Account > My Account > Billing section then set your account back to Starter plan then it should cancel your active subscription.