First thing would be to determine who you gave your number to which in turn, gave it to them.
Crooks got my cc number by uploading a virtual skimmer onto newegg.com. When I bought something, there was a page that wasn't normally there. I thought it strange, but the transaction went through, and it was only until I saw thousands of dollars of transactions that I knew I'd been defrauded. My cc company had insurance or something, and zero'd them out.
So, sometimes it's not even your fault.