My mac got hacked - what to do?

I booked a hotel and payed though the internet and it got accepted. But when we arrived to the hotel, they didn't get the money. And while I was on my vacation, someone used a lot of money from my creditcard. So I think that the hacker got my creditcard-informations though the booking of the hotel. But now what? I don't know what to do now. How do I know if the hacker still doesn't have connection to my computer? Do I have to install a virus-program or something to make my mac safe? I haven't done anything yet, but is my mac safe now? I don't know if I dare to buy anything from the internet again... Hope you can help me...