How do I resolve an unauthorised charge for a free trial renewal on my Apple Account?

I got a free trial for a month and completely forgot about it after a week, but now I cant download apps or buy anything and I saw it was because I owed money for the free trial renewing and now I dont really know what to do. Do I just change payment method so it tries to charge again and then request a refund?

[Re-Titled by Moderator]