If your debit card was not lost or stolen and a merchant just charged your debit card without authorization then you should contact the vendor first and let them know of the unauthorized debit card charge and see if they'll resolve the issue and refund you first.
Usually the company will refund the money back to your card without problems.
If not then contact your bank and put in a dispute on the debit card and then the bank will look the case over and decide if you'll be refunded the money and if so the bank will do a charge back which will cost the merchant money.
If your debit card was stolen or lost and was used without permission then you need to contact the bank and have them issue you a new card and stop the use of the old one.
If you notify the bank then you'll not be responsible for fraudulent charges on the debit card.
You should also report the stolen card to the police as well since it will be in their records should someone make unauthorized charges on your debit card.