A basic intro by W3Schools is here: https://www.w3schools.com/css/css_rwd_intro.asp
It will give walk you through the rationale and the process step-by-step and build your know-how progressively.
If you’d like to dive right into media queries, here it is:
https://www.w3schools.com/css/css_rwd_mediaqueries.asp
And for details on media rules, here goes:
https://www.w3schools.com/cssref/css3_pr_mediaquery.asp
If you’d like to dive right into creating a reponsive menu:
https://www.w3schools.com/cssref/tryit.asp?filename=trycss3_media_menu
