Resize your browser to view where the website naturally breaks based on its content.
The dimensions at which the layout breaks or looks odd become your media query breakpoints. - Within those breakpoints, we can adjust the CSS to make the page resize and reorganize.
See this article by Interneting Is Hard for detail.
The default breakpoints are chosen according to this diagram by CodeCademy. But these breakpoint values should depend on each webpage. The best practice, according to this CodeCademy lesson, is as follows: