How do you block porn from appearing on your Mac?

I need to block all porn contents from my Mac. My young friend uses the computer and I notice that when she is logging out, she clicks out of porn sites those are opened and tells me she doesn't know how they got there. The question I have is can porn sites pop out of nowhere and show their contents out of the blue? Can I get the internet filter for Mac to block unwanted sites? Please help and excuse my ignorance in this. Thank you.