It depends on the status of your current subscription. If you're still in a trial period, you will keep your remaining trial regardless of when you decide to activate your subscription. This means that even if you decide to go on and activate your subscription plan by entering payment details, you won't get charged until your trial ends.
However, if your trial has expired and you have lost access to your help center, you will get charged immediately after filling in your payment details and buying a subscription. Then you will immediately regain access to your help centers.