Not a great player or even an especially good one, but I feel like I've managed to improve a lot over the years so I have some thoughts about this.
The number one most important thing imo is to pick a format and really dig into it. At a beginner-intermediate level, there's a lot of learning you need to do, both in terms of raw mechanics and more conceptual stuff like team archetypes, prediction, managing gameplans, etc. The conceptual things are important to learn but it's very difficult to think about that stuff when you don't have a baseline familiarity with the format. So I would recommend picking a format you enjoy and then looking through all the resources you can find -- VRs, analyses, teambuilding guides, team dumps from good players... ADV is a really great choice for this, there's an embarrassment of riches here. Once you have a mental model of the most common mons and lines of play, it's a lot easier to improve fundamentals like identifying key targets/wincons, predicting your opponent's team, etc. Those skills are a huge component of player skill in Pokemon, and they're also much more transferable between formats.
There's a tournament called ADV Revival going on right now and I've personally found the
video commentaries to be both entertaining and informative. A lot of the matches have some weird teambuilding choices, but there's a good mix of players from different skill levels, and the commentary does a good job of calling out what's normal vs what's spicy vs what's just kind of crazy. They're fun to watch and I think over the course of the series I've learned a bit about prediction and team comps in the ADV OU.
Other misc things that might be worth stating:
- Ladder games and tourney games are both good practice but they encourage slightly different things. When you're going to play someone in a tour it's expected that you might look at their previous games and get a sense of their habits, which can incentivize you to bring different things. Even if you don't do that, a team that flounders horribly 30% of the time is just a lot more viable in a bo3 than it is on ladder.
- If you're iffy about whether something kills/outspeeds/whatever mid-match, it's normal and expected to just look it up. /calc is right there. Not saying you should do this for every move, but it's worth doing when you're up against something weird and the answer is important. Eventually you'll start to remember these things, and knowing them is a skill in its own right.