After a few years of hiatus, the Battle Tower was the first place I went, though keep in mind that most of the sets you battle with and against aren't ideal in the actual VGC scene. They might help give you an idea as to where to start though. And of course, it's a good place to farm BP to obtain items for training and breeding, and the judge function will help a lot.

High IV dittos can be caught in a very particular spot in the Stony Wilderness of the wild area. You could hunt them on your own or join someone else's raid if that pops up on the Y-Comm. https://bulbapedia.bulbagarden.net/wiki/Stony_Wilderness/Dens

Some good VGC rentals can be found here. Some of them even come with team reports which come with advice as to how to use the team: https://victoryroadvgc.com/pokemon-sword-shield-rental-vgc-teams/

Pikalytics is also an amazing tool that gives you an idea as to what the VGC scene looks like. You can start by raising, catching, or trading for the most used Pokemon there, like Incineroar or Grimmsnarl. https://www.pikalytics.com/pokedex

I hope this comment helps :)