On Smogon, you'll learn most from RMT and the analysis or C&C pages. Other than that, I often read and memorize stats and relevant damage calcs. Battling itself is obviously important for practice, but that kind of outside studying helps a lot for commentary. Good luck!
No problem, thanks for the comments! I don't actually do any prep work; if I seem knowledgeable, it's just because I spend a lot of time reading and studying the metagame. Knowing technical facts is important when it comes to competitive commentary, but it's just something you improve on over time.