Hello,
there is nothing special involved, the tournaments are simply posted here on the forum. If you want to participate, it is enough to simply reply to the specific topic and say that you want to participate, making sure as best as possible to be there on time, and one of the organizers will invite you to a table. The draws are also usually posted in advance so you can know who are you facing.
As for hosting, usually it is best to find a few people willing to help you. Before that however, you should choose the game, the exact rules and the format for your tournament. This includes things such as rules, number of players required, as well as number of tables, or how players will be eliminated, as well as the time and date for the tournament itself. With all of that ready, you can feel free to post a new topic in the tournaments room announcing your tournament and waiting for people to sign up. Usually, you assign a few people who you trust as the table masters, so they can watch the games and submit the scores to you after they are finished. If you receive more players than you need, it is always a good idea to keep them on the standby list, this way you can have replacements in case someone does not turn up.
Hope that helps. If you have any specific questions, you can feel free to ask, and good luck.