i think i have a virus on my mac book air

I know I have a virus on my Mac book random sites keep popping up while I'm searching things for example when I go to YouTube a site pops up saying "you have 14 viruses please click here for help" or when I'm searching stuffit goes to a completely different site that looks like google but a much older version and my Mac book keeps shutting down by its self. all I want is for these random sites to stop annoying me and not to feel like I'm being watched every second I open this

thing!!!

info: MacBook Air version-11,6

bought it maybe 2015-2017