popup adware virus

Hello,

The following message came up on my screen: "Dear Apple Customer, You have been redirected to Apple Support page because your Mac is infected with Popup Adware Virus due to download's from untrusted source.....etc." It looks fake. I did something recently, don’t know what, that has caused ads to pop up and has shifted me from the webpage I want to an advertising site. It may have been a phony Adobe Flash Player upgrade that I foolishly requested and which I am repeatedly offered. Any way to kill this thing? Thx. CP