Yay! I love Showdown!
First off, I recommend doing Random Battles when you first join. They will give you a good sense of what movesets and items for a Pokémon should be. They also give you a taste of competitive battling without the pressure of making your own team. Do this a lot. You can figure out your favorite Pokémon and you can find out their counters so you can prepare. Of course, before you even sign up, try to get as much knowledge as you can about Abilities, Items, Type Matchups, Natures, and the strengths of certain Pokémon. If you want to use Accelgor, for example, (one of my favorites) you wouldn't use Drain Punch because its a Special Attacker.
Next, I highly recommend studying tiers. If you bring, lets say, a Flareon to Ubers, it might be good, but its probably not going to do well, as its tier is much below Ubers. To study Pokémon and their tiers, I recommend going to the Smogon strategy Dex (here's the link): https://www.smogon.com/dex/
After you do all of that, you should go and pick out a tier (your format) (for example, Gen 7 Little Cup (LC)). To start building your team, you should take all of the knowledge you learned (please note that this site has moveset threads that can help with movesets) and create a team. Pokémon should have coverage along with STAB (Same Type Attack Bonus) moves, unless they are meant to tank/wall, not to attack. If you have a Starmie, for example, Ice Beam is a great choice for covering its Grass weakness. Try not to overlap types to much unless its a monotype team. Have Pokémon that can counter counters to your other Pokémon. Set their EVs and IVs as desired, and there you go. You can now start Showdown!
Hope this helps!!! :)