Malware on my MacBook air

I got a warning today that I had visited an infected site. Since then, I've started getting banners warning me that I've picked up a virus and to "click here to activate antivirus". It is clearly fake but I don't know what to do about it. I have Malwarebytes, but it is not detecting anything.

So far, the banners are merely annoying, but I'd like to clean my mac if possible. Any ideas?