How do I stop a charge from processing?

Hello, I'm wondering if you could help me understand how to stop a charge from processing. I recently made a purchase using my credit card, but I've realized that I no longer want the item or service. Is there a way to cancel the transaction and prevent the charge from going through? I'm not sure what steps I need to take to ensure that the charge doesn't go through, and I'd appreciate any guidance you can provide. Thank you in advance for your help.