https://play.pokemonshowdown.com/
This is the thing I'm talking about. On Pokemon Showdown, there are different formats (or tiers) you can play, like Overused (OU) Underused (UU), etc. You can build teams for those formats. Each format also has a Viablity Rankings that have which Pokemon are viable, which is useful for building teams. My advice is to pick a Pokemon you want to use, learn about the format its viable in, and then build a team. For example, if I really like Talonflame, I would learn about Gen 8 NU since it's the tier Talonflame is viable in. There are formats called Anything Goes, but I wouldn't recommend starting out with Anything Goes (AG) because it's a legendary filled tier, so if you don't like using legendary Pokemon, the AG formats are not good starting formats. Hope this helps some.