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
I have read the text 4 times now, but I neither see a question nor understand what you are talking about. Could you please rephrase?