如何在CSS中设置网站以对不同的分辨率做出反应？

For a college project, i need to take a website that my teacher gave me in HTML, and set it up so that if the resolution is cut down to a max-width of 640px, i need to take out the picture and nav bar at the side. I'm just unsure as to how i program it in CSS so that i can hide sections when 640px is met? Sorry if this requires alot of explaining but we dont have a textbook for web design right now and i cant find any examples on the schools online database that i could refer to, to figure it out thanks in advance For a college project, i need to take a website