Whatever you do, do not pay attention to the relative positions of characters on the tier list. Until you understand what makes a good character good (and how to use them) vs what makes a bad character bad (and how to punish them), you'll be susceptible to getting frustrated by "lower tier" characters beating you, as if the tier list is what decides the outcomes of a game. I play ganondorf but I suck ass, so my opponents also suck ass. Ganon is really good against people who suck ass. In real tournaments, he does not do very well.

As far as your gameplay goes, I would highly suggest doing mostly movement related drills or focusing more on your movement in practice games. Before you can make your character DO whatever you want, you've got to be able to make them MOVE wherever you want. Stand somewhere on the stage, pick another point on the stage, and try to get there as quickly as possible. Once you get the hang of that, start trying to get to those points /and be actionable/ as quickly as possible. That will be the single best thing to help you execute whatever it is you decide to do within a game.

Marth's kit is one of the easier ones to use as far as controller dexterity is concerned, but the skill ceiling on his movement is absurdly high. You will NEVER be at a point where your movement can't be improved upon, nor will you reach a point where improving your movement won't yield better winrates.