I've never took part in a tournament, but from my understanding it should be something like:

Make sure to be good in your fundamentals and mind game (universal skills that can be applied to any character): movement, spacing, mixups, neutral, advantage and disadvantage management, shield management.

Choose a character to main and learn what are their best stages, and why. Learn any notable techs the character can take advantage of (like IDJ for falcon or slingshot for lucas). Learn bread and butters and kill confirms and the most common mistakes people make with your main (if you use bayo don't side b to get out of ledge).

To do all that stuff you can watch tutorials like the Art Of series, and Frenzy Light's Bread and butter combos series. Also series like sillintor's "I practiced X for 3 days and entered a tournament" could give you a good idea of the general approach. Follow the top players of the characters you main and analyses of their games, learn the interactions and matchups.

And of course, while doing all that stuff, play the shit out of your main(s).

And don't worry too fast about your tournament results, focus on having fun and getting better and the rest will come.

Please correct me if I made any mistakes.

Also remember that while you will have peaks and plateaus, there isn't an upper limit on getting better really. As such, there won't really come a moment where your "good", just moments if getting better.

Enjoy the process and let it happen, don't beat yourself up when you need to take breaks.

I am nowhere near playing on tournaments so take my advice with a grain of salt.

Pick a main character (for now), does not matter if it is the best in the game but it should be at least B+ in the Tier list . It's more important that you feel comfortable with the character and have fun playing it than if its S Tier. Then go on YouTube and find out the basic and advanced combos for this character and practice them in practice mode until you can execute the almost without thinking

Disclaimer, some characters do not really have a combo game while others just go bonkers in combos. For example Lucina is a good character without a crazy combo game. If you have a character like this the next step is even more important.

Learn the spacing of you character, how to approach and how to keep control of center stage. Those are usually rather vague concept, but if you go into matches just thinking about one of them you will progress there.

Lastly, watching more YouTube. Thereby you learn a lot of match ups, characters and their usual habits without playing yourself on hours in crappy wifi. I can recommend poppt1 as one of my favorite, who also has a series where he picks up different characters to learn them for tournaments.

Good luck, have fun

Find a training partner to play against in person is the best way. Until then, focus on movement and execution.

That said, just go to an event and play people. No matter when you start going to events and no matter how much practice you put in beforehand, you are extremely likely to got bodied. And that's 100% okay. Pretty much everyone has gone through that same experience. It doesn't have to be a tournament. A simple smashfest near you is perfect for this. You'll lose most/every game at first, but you'll be able to talk to the people beating you and get advice so you'll improve much faster.

Just go in with an "I'm new but looking to practice and learn" mentality. Most smashers I've met are pretty receptive to that attitude.