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.