When you start playing Pokémon Sword and Shield, you will need to decide who will be your starting Pokémon. You can choose from Ouistempo, Larméléon and Flambino. Fortunately, you can also get the ones you don't choose by taking advantage of a few in-game features.
Your currency to get the other two starts will be your go, as the idea is to trade with The people who are looking for the Pokemon you have chosen in exchange for theirs. Because you can't trade your start if you want to have all three, you'll have to get your hands on a ditto.
The same can happen with your starting Pokémon, producing Pokémon eggs that will give another Pokémon the same species as your starter. After that, all you have to do is find a player who is researching your Pokemon and also raising their spare starters for trade.
To breed Ditto and your departure, head to the Daycare Center on Route 5. Give them your Ditto and your starting Pokémon for half an hour, then come back. If it yielded an egg, you add it to your party and play the game. To be able to hatch the eggs, you need to set the travel distances. A simple method, therefore, is to use the fastest method in the game.
And that's it, it remains only to try to find a player who is looking to trade. I would suggest starting in the comments section of the video linked above. Hopefully this will become a good hub for people looking to start trading with each other.