Strategy
Marth has highly variable neutral plans and punish trees per matchup and as a result a general strategy page is of limited use compared to more matchup-specific resources. Despite this, Marth has several options that are almost universally extremely strong.
Dashdance and Dashback
Marth's dashdance is arguably the best in the game, with high speed and an animation where he adopts a very low position to the floor that allows him to dodge many attacks and grabs effortlessly. Every matchup in the game largely revolves around Marth abusing his excellent dashdance and stupid dash animation to safely threaten space and safely throw out moves.
Efficiency
Marth players must be efficient in their punish game. Dropping edgeguards and combos, or even just comboing without finding ways to convert into a kill move or edgeguard scenario will result not only in wasted openings, but will make Marth's job significantly harder. This is because most of Marth's moves are counterintuitively significantly less rewarding when the opponent is at high %, resulting in the bane of low-level Marth mains: Marthritis. Thankfully, Marth has an excellent set of tools for consistently making the most of his punish opportunities.
Kill Moves
When aiming to kill outright with Marth, you are hoping to combo into one of three moves: fSmash, dAir, and upB. How you combo into these and which ones you will likely combo into is highly variable on matchup, with the only "universal" setup being the Ken combo.
Edgeguards
Marth has excellent offstage game and tools to cover ledge and can thus very effectively and consistently close out stocks once his opponent is offstage, particularly if they are without their jump. Runoff fAir, dTilt at ledge, and strong upB are three particularly strong options but, like almost everything with Marth, your plan of action will be highly variable with matchups.
Grab
Marth's grab has huge range and his excellent grounded movement allows him to get grabs with unmatched effectiveness. On top of this, his throws are extremely rewarding, with dThrow and fThrow forming the basis of many DI mixups and setting up techchases, and uThrow putting the opponent in a terrible position or starting a combo outright.
Crouch Cancel/ASDI Down
Marth is a prime abuser of crouch cancel and ASDI down due to his aforementioned excellent grab game. ASDI down can also be done while dTilting to make a safe poke even safer. However you must be wary of characters with rewarding multihit moves such as Fox's dAir, and meteor smashes and spikes such as Captain Falcon and Falco's dAirs respectively.