Media Queries Question

So I am a new to coding, and I was having a think about how to make website responsive. I understand that you use media queries to determine the browser width, and depending on that show the site one way or another.
However, I was wondering if there is a standard practice for this? As in I am assuming there are as many deviced widths as there are clouds in the sky. Is there a standard way to tackle this?

Also, when testing a site, how do you go about it to see how a website looks on a mobile device?

Thanks in advanced!