"Hypothetically, let’s say you started with a large, 1020px wide site. Check it out on the largest screen you can get your hands on and make sure it looks great. Now drag the window and make it smaller until the design gets ugly. There’s your first breakpoint. Set a media query for that point and fix everything that you need to address. Once you’re finished, grab that window and find the next point of ugliness. Repeat these steps until you’re satisfied with the range that you’ve accounted for. "
Joshua Johnson