I know there are some questions similar to this. But still, I don't understand how it works.
Recently I heard that there is a tournament held, and stuff like sumwun and Shining Yin winning a match. I always wanted to participate in a Tournament. I read from other questions that you have to reveal your showdown name and timezone. But where and how do I do that? And how are the formats decided? Do we battle on the PokeBase showdown server? Also, when will the next tournament be held?
The tournament you read about is taking place through this thread. It has started already so you can't participate in that particular one, but if there are any more tournaments in the future you can definitely play in them.
It's worth mentioning that the way that particular tournament came about was a bit different from how they normally work; traditionally, we would make a thread that announces the tournament and provides details about it, ie. rules and the number of participants. (Which tournament is chosen is determined by whichever post is voted the most on this page.) Then, users post answers on that thread stating that they want to participate -- this is where you would mention your Showdown username and timezone so you can arrange times to meet and battle. There's also this old thread which you can post usernames on; it might be good to bring this one back if we start doing regular tournaments again. I'll makes some edits to it so that it's up to date.
So basically, just watch out for new meta posts that announce tournaments and make sure you post an answer stating that you'll participate before others take your place. Here are a few super-old tournaments that you can use as an idea of what to expect. These posts only have one answer, I think because we would hide each participant's post when they were eliminated from the tournament.
How to change your username
How to add a user avatar
Why was my post hidden?
The points system
Flags vs Downvotes
PokemonDb Showdown server
Report minor site errors
Name Origins corrections
Location guide corrections