You probably think you know what I'm going to say here, don't you? (Hint: Don't be fooled by the photo above!) You think the answer is a paper bag. And that's not a far-off guess, but it's not correct. (I told you not to be fooled by the photo!)
Paper bags are really great for making microwave popcorn, but if you make popcorn as much as we do in my house, all those paper bags are going to get pretty wasteful. Even if you reuse them a few times!
If you want to go the paper bag route: How To Make Popcorn in the Microwave
The Best Microwave Popcorn Popper — Shopping