You can find tournaments in a number of ways, one way is to add the feed from this server
https://discord.gg/CbM6bHH (Splatoon Competitive Feed) to a discord server for your team and it will link you to different tourneys that you can play in (those tourneys should have instructions on how to sign up, but generally you join the discord for the tourney, sign up on a website like battlefy (the links will be in the server), whoever is the captain adds all the team (doesnt have to be the actual captain, just whoevers managing the team on the website), everyone makes sure they have there IGN in the system (and splashtag number; many ppl forget that one), and then you check in a little bit before the tourney). the TOs will seed you and you will match against teams!
I will note as you seem to be low ranked (if your signature is accurate) most tourneys are ranked modes only and most players in tourneys are S+, so i would go into tourneys expecting to be out of your depth. If you are interested, they are harder to find and less frequent but there are tourneys with low skill caps (
Under Pressure has one that is capped to A rank but you have to enter alone) if you are willing to find them.
good luck and I hope you have fun competing! Its very fun :)
I need to play more tourneys when i get time