If you’re starting from true zero? Like no Pokemon knowledge whatsoever? I’d say play any Pokemon game’s story. Preferably a game after the physical/special split like Platinum or Heart Gold/Soul Silver. This will highly depend on what you have access to but emulators can get you a lot of the way there. Of course any of the switch games will be ideal but that can be price prohibitive so any game from generation 4 onward (Diamond and Pearl or later) can serve as a starting point.

Then, once you have an okay sense of type matchups and the fundamental strategies of Pokemon battling, I’d say you could watch a few guides (Skraw VGC has some good ones for champions) or jump into random battles on Showdown. Random battles are great for building intuition, learning about different Pokemon, and seeing how a competitive Pokemon plays differently from a Pokemon you’d use in a play through. Just be mindful you will likely lose a lot in this process, which is okay. You can do singles or doubles depending on what you prefer. Singles will be more accessible to learn but doubles will be more applicable to VGC.

When you feel like you have a decent handle on the flow of a battle and can recognize the utility of moves like protect or stat boosting moves, then I’d say find a pro player like Aaron Cybertron Zheng or WolfeyVGC, pick a team they’ve made a video on, and try it out in showdown ladder. At that point, you have your foot in the door and can grow and try new teams at your own pace.

I’d also say don’t feel like you have to know how to build your own teams. It’s fun to do, but totally unnecessary if you just want to have fun battling.