As someone who only ever played tournaments at the local level through the tail end of PM and the entirety of Smash 4 (And sadly only a few Smash Ultimate so far), here's a few general things to consider:
1.) Your practice schedule/Practice Partners. Even if prizes aren't your concern, I assume playing well and placing well in general are still a goal. Practice seems like a 'duh, of course' type of suggestion but it's something you shouldn't skip out on. Even just running basic muscle memory drills is important. Also find solid practice partners. Ideally folks who are 'above your paygrade'. This not only gets you experience with better folks but helps you start to be better in general and learn your areas common match ups but allows you to study their habits. A bit of mutual espionage in that regard.
2.) Define to yourself what 'Professionally' means. Pro as in playing at something like Evo and big majors or 'Pro' as in a local "Boss" that gives folks a run for their money. Because that will help you focus your goals and expectations. I'd assume you want to be a local pro because you'd probably not want to be going to majors if prizes mean little to you. Unless you have expendable cash that can cover travel, lodging, entry fees and other expenses and possibly bustering out day 1 is no big loss monetarily.
3.) Train Tournament Nerves. Just by attending locals you'll overcome these, if you have them. But they can be a real run ender. Some play music to help. Also dealing with spectators or crowds can be difficult, especially when people have no boundaries in local settings, often crowds standing RIGHT behind you and often bumping your chair. Truly one of the most annoying things about tournaments, Smash ones in specific. Etiquette can be pretty bad.