I got a message that my computer has virus and I need to buy antivirus software from them to remove it. I paid and after that realized I've been scammed!

If you have antivirus software and set it up correctly, then these kind of scams wouldn't work on you.

Dispute the charge with your bank and then download a free anti-virus program, run it often, and when you get a notice that you have a virus. You can still get viruses with an anti-virus program, but running it should remove any virus you get.