If you want to join a tournament, go back to the forums page where you posted this. If you look at all the posts, there are a lot of community hosted tournaments that you are able to join.
From there, do the following:
1. Find a tournament that matches the mode you are attempting to compete in (osu!, osu!taiko, osu!catch, and osu!mania)
2. Make sure that you are in the rank range for the tournament (should say in title. Do NOT worry about BWS for now.)
3. Look through all the information in the tournament and make sure that this is the one you want to participate in.
4. In the post, there should be two links: Discord and Sign Up. Join the Discord and then Sign Up for the tournament (Note: Some servers have the sign up link in the Discord and not in the forums post. If you do not see the Sign Up option, check the server.
4a. If this is a team tournament, there should be a freelance channel where you can find teammates.
This is the technical way on how to actually play in a tournament. This isn't "How to be good in tournaments." Personally, I don't suggest you to play tournaments at the moment until you improve on every single mod. Search up tutorials on how osu!tournament works and you'll get the hang of it.
Good luck on your road to becoming an osu!champion!