There's no exact way to start at mapping since no one can force what you should map and what you should not map. You will learn everytime you map and get feedbacks. I'm not recommend pishifat's videos this time when you don't have some basic knowledge of mapping yet, especially mapping terms. Just ask what you don't know here, or PM who you trust most for answer, as long as they're willing to answer for you.
tl;dr just map more, and learn by yourself