That's going to be different for everyone, since, what works for one may not work for another and vice versa.
It's generally best to start in a niche you are already well versed in. for instance:
If you have worked in the coffee business most your life, coffee related domain will be easier for you to work with, understand the market dynamics for, and communicate about with a potential buyer.
If your favorite sport is basketball, you attends basketball games and collect basketball stuff, then basketball related domains may be a better option for you, again, because they will be easier for you to work with, understand the market dynamics for, and communicate about with a potential buyer.
By starting in niches you already know, it helps you navigate the learning curves easier when it comes to investing into domain names for the purpose of reselling.
If you start registering/buying domains other people do, out of the gate, without any knowledge in the niche they are in, you're setting yourself up for a very hard time trying to resell them, you'll need added research to understand them, and you'll potentially wind up with a big loss in monies you spent.
Even going for niches you know, be sure to read up on how to manage, monitor, evaluate, market and resell domain names.