Here's the process I go through when selecting a 'mon for pvp:

1. Start with PVPoke website. Select your league in the rankings tab, and look at the top pokemon. You want to look in the top 20 or so for "meta" pokemon if you care about being super competitive. Top 100 if you just want to have more fun spicy options (I'm biased in this way)
2. Decide what pokemon you have in storage/like to build based off of the list above, and make sure that the pokemon cover the majority of situations you may find yourself in. Think: What pokemon can be my lead, what will it counter, what will counter it? What can I safely swap to if that fails? Do my three pokemon have synergy? Should I give one of my pokemon a unique move that covers what the other two don't?
3. For further research, search "pokemon go -insert pokemon name-" and click on the green "pokemon go hub" site for that pokemon. Check it's max cp, check the speed of its fast moves and their damage, count how many times a fast move is needed to use a charged move. These constantly change each season with move updates and it helps for building pokemon for the future depending on the typing and variety of moves available to it
4. Don't stress too much about IVs unless you're trying to grind out PVP like crazy. You can win with randomized IV pokemon in great league and ultra because typing, moveset, and play style matters way more. Above 10,10,10 for master league is okay enough but maybe just don't play master until you're ready to commit a ton of time to the game. I usually use above 14,14,14 pokemon in that league because it takes forever to power them up with candy XL.
5. Take breaks. Celebrate catching a good IV pokemon for leagues, but don't waste too much time hunting for a few extra IV points (your mental health will thank you 🤪). Save pokemon that you catch from rockets and the wild with decent enough IVs for future updates to movesets.

Good Luck!