Why is my mac saying i have virus's,worms etc?

I recently bought a new macbook, and i have only had it for a few days. All of a sudden, popups keep appearing saying my macbook is critical because it is infected with virus's, worms, malware etc. I downloaded the protect mac anti-virus from apple, but it scanned my macbook, and it says i have no threats, so why do popups saying i have virus's, worms, malware etc keep popping up? Also a random gay porn website keeps popping up as well, i'm so sick of it, and embarrassed as my family use my laptop. What can i do to get rid of them?