Welcome to e4, BumBumDance!
There is no right or wrong way to begin collecting Pokemon. My one piece of advice is to look at many different sets, promos, language-exclusive releases, etc. to get an idea of what you might like. It will take some time before you feel fully confident about your collecting interests, as they will morph with experience.
What makes opening a $400 modern box “worth it?” If you are asking about how much fun you’ll have from pack opening, that is only something that you can answer. In terms of value, no - you will almost always lose money opening modern product. Only a small portion of lucky pack openers will make the value of a box back, and that is typically contingent on getting lucky with card grading/selling.
Hope this helps!