The first amount is due immediately on purchase; access begins at the same moment. After that your app store charges monthly and renews the subscription automatically.
The charge date follows the day you bought, not the start of the month. Your store shows you the next date: on Android under Payments & subscriptions → Subscriptions → Citily, on iPhone under Settings → your name → Subscriptions.
If you cancel, the next charge does not happen, but the month you have already paid for runs to its end (see Cancel your subscription).