If you unsubscribe before your FREE, 1 week trial ends, you will not be charged.
If you are past the free week, you could try to get a refund by sending a request to Apple: Request a refund for apps or content that you bought from Apple.
Note: Apple manages refunds and we don't have control on whether you will get one or not. If you don't do this often and you are within a week or so into your subscription, you should have no issues.
To cancel your subscription, see this link: How to cancel a subscription from Apple