I would honestly start honing your skills on a place like Showdown, which will help you automatically get an EV and IV spread, and a accompanying Nature, once you choose and Pokemon and it's moveset. Start by looking at a place like Smogon and look at the tier list. You can also do this in Showdown, as you can previously create a team based solely in one tier list (OU, PU, Ubers, LC, etc.)
I wouldn't go into SwSh competitive blind, because you're going to struggle, and if you're anything like me; struggling will lead to anger and disenchantment, and you end up quitting the whole thing altogether.
While Showdown obviously doesn't have a Gen 8 metagame yet, it's beneficial to you now to get your feet wet and dip your toes in right now as opposed to later, or when Gen VIII drops in November. I know, you're going to playing in the tail end of this current Generation's meta, but playing now will get you accustomed to the intricacies of competitive Pokemon.
You can also use Showdown as a sort of sandbox mode, once you get familiar with how the competitive side operates. Create teams, and move sets and then breed in Sword and Shield once you feel ready to actually play Pokemon competitively.
This isn't easy, you're going to go through rough patches where you'll find out how nasty RNG can be when she's not on your side. The thing is, to keep your chin up, and remember that it's only one match; the RNG can easily swing back in your favor in the next battle.
