In most cases Apple’s default has auto-renewal turned on and you pay automatically.
If you don’t want this, go to the Settings (Settings > your_name > iTunes & App Store Apple ID > subscriptions) and disable auto-renewal.
If you do not have enough money in your account/if your linked payment methods fail to provide the money, Apple will send you a few emails reminding you to update your payment method. If you choose to ignore, your subscription will automatically not go through.
To be on the safe side, follow what Anton has said.