sudo pico hosts

I would like to be able to block web sites on a "per site" basis". A search of the forums produced results using the Terminal and sudo pico hosts command. I am not familiar with Unix, therefore I am hesitant to go this route simply out of concern that if I make a mistake it will cause major problems and my child will not be able to access the web content he needs for his school work. Is there a way to make a back-up copy of the "Host" file before making changes and if so how easy is it to restore using that back-up? Thanks in advance.