someone has been using my debit card fraudulenly on fortnite

First, do anyone in your household plays Fornite? This is important information requesting charges not made by you:

This category covers purchases made by someone else using your payment information (either on your account or their own). Please read the following notes before continuing.

If you don’t know who made this purchase:

• If you have compared your bank or credit card statement with your Xbox account purchase history (as well as the purchase histories for all family members) and cannot identify the source of the charge you are seeing, please request
  a call from Xbox Support so we can remove this payment instrument to prevent any further unauthorized charges.
• You will need to contact your financial institution for a refund.
• If you see charges on your Microsoft account or your child’s Microsoft account that were not made by a known person, contact your financial institution.

If you do know who made this purchase:

• If you know the person who made this purchase, see the article Prevent unauthorized purchases on Xbox One for tips on how to keep your
  account secure and prevent unwanted purchases in the future.
• To help us better protect your account, we’ll need you to tell us whether the person who made the purchase was:
  1. your child;
  2. another child;
  3. an adult in your family;
  4. a friend or other known adult.

You can provide additional details about who made this purchase, and whether the purchase was made on your account or theirs, when you fill out the refund request.

Please don't forget to contact your bank to let them know about your situation, try to get a different debit card as well.

Mostly all of the solutions here are good. As stated I would remove the debit card from the account then contact the financial institution to report the card compromised to get another one and while doing so contact EA to see if you can get the charges
credited back to the account and if that doesn't work file a dispute with your financial institution for your money back. Also finding out how the person gained access to your card info may be helpful especially if the cid/cvv code was used to make the purchase
because it may be a source closer to home doing it rather than an outside source like a random stranger.

Yes, change your password and remove that payment information from being automatically stored on your console. Also, you can contact your bank to either dispute the charges and report them as fraud, or to cancel the card altogether and have a new one issued
with new numbers.

You should immediately call your bank and get your debit card blocked and cancelled. It'll also be a good idea to change the password of your Epic account.