Need Help With Media Queries

I'm a web developer noob and need some help with media queries. I'm working on a project for school and we're building a site like http://www.itemcycle.com where people can sell their old Apple products. On that site when you grow and shrink the page it auto-adjusts to fit the type of screen.

I've Googled around and landed on the fact that I need media queries in my code. What size media queries am I supposed to use? Aren't there way too many devices to target? Thanks guys!