In media Queries whats the difference between mobile first and desktop first development. How would you layout the media queries?

Mobile first and desktop first are2 approaches to writting the CSS for you website. 

In mobile first approach, your base css is for the mobile view and you add css with different breakpoints moving up in the size (tablet and desktop)

The desktop first is the reverse. Here your base css is for the desktop view, and you add the necessary css to change the UI for the mobile view with the different breakpoints. (tablet, mobile).