How can I get an app refund?

I canceled my subscription. It was a trial that I didn’t even expect to be charged for. I made sure last night before I put my new credit card information in that the subscription was canceled but I still got charged $39.99 today for subscription I did not ask for. Can you please help me with a refund because I do not want this app, and I certainly didn’t sign up for one year

[Re-Titled by Moderator]