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Which format are you looking to try? I'd recommend checking out somewhere like Smogon for some guides to basic competitive battling but VGC is completely different than something like Smogon Overused so you'd have to make sure you are reading up on the specific format you want to get into.
It's all about knowledge and making middle-ground plays, so I suggest just playing a lot, and reading up on what the user above me has noted. I'd add to that by telling you to focus on a specific tier you want to play at first. You can always branch out at a later time, but it's good to have a "main" tier.