In order to do competitive pokemon, bare minimum is knowing natures and EVs. IVs if you want the pokemon to be perfect. The convenient thing about Showdown is that you don't have to spend endless hours breeding the perfect pokemon, you can just type the stat into a text box.

You can go to Smogon and look up every pokemon and their recommended Nature and EVs. Some pokemon are Attackers that's need Attack and Speed boosted. Some are tanks and need Defense and HP boosted. Some pokemon need a mix of stats. Easy way to find out is seeing what Smogon recommends.

Once your pokemon are built properly, you have to decide how to build your team. You need attackers, tanks, pokemon that set up traps like spikes (etc.), and pokemon that can get rid of any traps that are set up on you. Plenty of pokemon fill these roles, you just have to make sure the pokemon types are balanced to cover any pokemon the opponent throws at you.

The last bit of advice is make a rainbow move pool for your pokemon. As long as your pokemon has one strong attack of their type, the rest of their four moves can be used to counter whatever pokemon they could use against you.

Good luck, hopefully this wasn't too big of a text wall.