Chiefly through experience playing the game. Reading will give you an idea of *what* is happening, but you might not get to the *why* until you've experienced it yourself.
With that said, Smogon has pretty good resources that will at least give you some things to look out for and help you make some informed decisions.
* Usage stats for Showdown (including movesets), which you can narrow to high-rating matches. Includes official formats (i.e. not Smogon), but obviously only data from Showdown:
https://www.smogon.com/stats/
* Smogon's strategy Pokedex, which is written by many experienced players and should give you an idea of each Pokemon's niche (if any) and how they match against each other. Also, explanations of how each battle format operates:
https://www.smogon.com/dex/
* Sub-forums for current tiers/metagames, where you can find viability rankings, speed tier tables, suspect test discussions and more that should keep you versed in what's happening in the game.
https://www.smogon.com/forums/forums/525/
* Closed forums for old metagames:
https://www.smogon.com/forums/categories/283/
You can also find advice on YouTube, especially on what to think about when you're playing battles. YT has a lot of bad information on it though, so you need to be discerning. The most popular videos are not always the best. I recommend Pokeaim and NathanLikesChicken for Smogon formats, but I'm not well-versed in VGC.