Ok cool, so we'll be trading through the y-comm menu. Open it and have it connect to the internet by pressing the + button.
We're going to be doing a link trade. It will give you the option to set a link code. When trading like you and I are going to, we'll pick a 4 digit number we both agree on so we end up linked together to trade. You can do a link trade with a random person (without setting a link code), but be cautious with those, people have been known to walk away with your Pokémon!
So! Let's trade! What would you like first? I have a full Pokédex in sword and almost full in shield.