Warning: Still trying to improve, not 100% on some of this.
Marth's strengths are his amazing neutral game, strong punish game on fast fallers, and the ability to keep the opponent out with his sword. His weaknesses are the commitments his moves create and his somewhat poor recovery.
You should become familiar with jump cancelled grabs, down tilt, dash dancing, and wavedashing. Those form the core of Marth's neutral game.
Become familiar with tipper locations on all platforms with up-tilt and forward smash.
Become familiar with up-throw and possible follow ups.
Never, ever use back throw.
Good matchups are fast fallers - Falco, Fox, Captain Falcon, where your punish game comes out strong and your neutral game suffers a little bit. Sheik plays an intense neutral game with you but has the advantage until a very high level. Peach plays weirdly against you, since she has a difficult time trading against your sword but she can kill you very dead if you let her in too many times. Puff is weird as well. You have a somewhat easy time against Ice Climbers since you can separate them pretty easily, but you can't grab very easily in this matchup. Marth is a 100-0 matchup.
Marth plays the neutral by staying grounded and spacing the opponent out with down tilt and dash dancing, punishing attacks and poor movement with grabs. He can do retreating neutral air or forward air, which are amazing and almost impossible to beat due to how much space his sword covers. His aggressive game suffers a little due to that commitment, and a lot of newer Marths will get into the trap of using forward air or neutral air to approach, get punished for it a lot, and struggle to improve. You need to try to control the ground and then force the opponent to take to the air, where your forward air becomes an amazing tool rising up since it covers the area above and in front of you in a few frames.
Marth likes every stage to some extent except Dreamland; in a 2-3 set you will ban Dreamland every time you win a game. Every other stage has amazing advantages for him, from Final Destination's lack of platforms to support his grab, up-tilt, and up-air based punish/combo game and to deny the opponent movement around his ground-based neutral, to Yoshi's Story/Pokemon Stadium providing very easy forward smash tippers on opponents who you can force onto the platforms. The worst stages for you vary by matchup - against Captain Falcon, for example, every time you win you will ban Dreamland and he will probably take you to Final Destination or Pokemon Stadium.
Battlefield tends to be the "most neutral" stage and you will likely play on it a lot because people will strike you to there, since you're banning Dreamland every time.
Be careful of Fountain against Falco and Fox because, although it disrupts their SHFFLs and laser pressure, your large body makes you very comboable; the platforms work well for you, however, so this stage can be a matter of personal preference. When playing against Fox and Falco players, if you can get in their head while you are chaingrabbing (also practice the chaingrab, it is one of your strongest tools in this matchup), you can down throw instead of up throw - at low percents, a missed tech and your immediate f-smash will likely kill them due to unprepared DI. Don't do this against high level players. Be aware that it becomes possible to up-throw into tipper on Fox around 60% - not sure on Falco percents (or if they exist).
Pokemon and Yoshi's Story make it very easy for you to kill opponents you can get onto the platforms, particularly space animals, but Fox will tear you up on both of these if he gets in because the ceilings are very low.
If you can master the tipper spacing on Battlefield you will get a *lot* of free kills on low to mid tier players as they shield above you. Since it barely pokes above the top of the platform, it becomes incredibly easy to shield poke.
Try to take Sheik to Yoshi's Story or Fountain since it invalidates a lot of her autocancel shield pressure near the platforms (she can't short hop under them) and lets you tech down throws a lot earlier - be aware there's a lot of times you can fair/jump out of Sheik's throws as Marth.
Final Destination varies from the best stage possible to the second worst stage depending on the matchup.
You don't ever go to Dreamland.
Or back throw. Down throw is the one that sends behind. Yes, this is unintuitive.
