You are going to need to provide more details. First you need to determine if the subscription is an in-app purchase managed by Apple or one that was made with the Developer directly. For subscriptions managed by Apple, they are cancelled here:
View or cancel app subscriptions on iPhone - Apple Support
And also seen in your Purchase History:
View your purchase history for the App Store and other Apple media services - Apple Support
For subscriptions managed by the developer, you have to cancel on their website where the subscription was started.
If you signed up for a Free Trial through the app on your device, then you would have seen the Authorization Screen with:
- Free Trial Length
- Subscription Start Date
- Subscription Price
- Subscription Length
- Message stating to avoid charges you must cancel the trial 24 hours before the subscription start date.
If you did not cancel 24 hours before the subscription start date, then you owe that amount that you had agreed to pay from the Authorization Screen. Since you said it "wants" your money, that sounds like the payment was not able to be processed and is in the Pending state. You cannot cancel a Pending charge, and that does not mean you do not have to pay. While you owe money, it is likely you will no longer be able to download any more apps in the App Store. You are also not able to request a refund on a pending charge.
