Have I got a virus?

You may have a virus but your iMac doesn't have one.

What site are you see this on?

Sounds more like a phishing attack with the small amount of information you provided.

Force Quit your browser

Hold the Shift key and relaunch your browser

Does that clear up the problem?

As Allan Eckert suggests, it is most likely a phishing attack attempting to get you to give up money or personal information for identity theft.