Okay, since this is your first local, one key thing is your mindset when going into your matches. For now, don't set any expectations for yourself other than you will take your matches and learn from them. Analyze how your opponent plays and consider how to either stop him or her from controlling the pace of the match in order for you to do so instead.
Make sure that your movement is at its cleanest currently because that matters so much for Marth with his dash dance and wavedash lengths, but don't beat up yourself too much if your movement is not super crisp in comparison to your opponent because your movement will improve from practice and experience in tournament matches (as long as you make the effort to of course).
Don't worry too much about highly advanced tech right now since it's mostly important that you try to learn and get an understanding of neutral game first. I'm not saying that the tech involved is not important; it's just not what should be the thing to focus on in matches because you can learn tech on your own and can practice it without any practice partners, unlike the other aspects of the game (except for what you specifically mentioned along with wavedashing/wavelanding, that tech is highly needed for any character at the beginning). Once you learn the neutral game better with exploiting openings or creating them, the advanced tech can only help your neutral game become better due to more options that you can naturally choose from at a fast pace.
There is one thing that can be said immediately: Don't let yourself become too desperate that leads to spamming a move to get the opponent to stop pressuring you. Things don't end well when this happens since it just makes it easier to get punished more due to being flustered, and your long-term growth as a player will be hindered. Instead, focus on what you would do to reliably escape punishes or pressure. If the method isn't working, you either need to practice it more or it actually doesn't work in the first place. For example, if your opponent is slow enough in starting or continuing a combo, Marth can use his fair in order to knock the opponent away so that he can regain his footing. Most importantly, when you end up in a bad position in any case, try to understand what happened that lead to it in order to try to adjust to the strategy mid-match or to adjust your gameplay in future matches when looking back.
One part you absolutely need to consider when in a bad position is your DI. You will learn how to use it more effectively as time goes on, but try to make sure that you use the correct type of DI to revert back to the neutral quickly or to just stay alive. Otherwise, things can happen like accidentally using combo escape DI against Falcon's hard knee and ultimately dying earlier.
Another thing to note: Your own punish game. Most players are only able to get 1-2 hits per opening, so don't expect yourself to take a stock per opening or even two openings. Your punish game will naturally get better with again practice and experience. That being said, Marth's punishes are largely based on juggling your opponent or setting up your opponent to get hit by a kill move or to get edgeguarded by Marth's amazing range.
I won't get into a list of combos here because it largely depends on: DI, percent, and the matchups.
This is most of the important stuff to consider at first. There is more to consider, but most of it ends up being more tech skill and details in regards to your micro-spacing and punishes on the whole cast. Every tournament you attend is a learning experience, but the first one is one of the most important in this aspect. Now all I can say is, have fun and get a great experience out of it.
