What calibur is this Marth?
You can't outspace an equally skilled Marth, so you'll have to play a more timing-based game. Snipe him in between aerials with Bair. Dair is a good move to use once inside his range, as it will pop him up and get you into his blind spot which is directly underneath him. Predict when he will UpB in close quarters, and be especially wary of it on Battlefield and other stages where he can land safely on an upper platform. If he does an aerial really close to you, he's probably trying to bait you into an attack; Marth is a slippery mother****er that just glides away majestically whenever he feels like it, so don't charge into his Fsmash.
At 0%, he can Fthrow into Fsmash guaranteed. Nothing you can do about it; you deserve it for getting grabbed at 0%
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He's also solid at edgeguarding of course, I recommend you recover high. Learn to time edgehogs so he can't recover safely. You can FIHL his Fair but it's obviously gimmicky and won't be the reason you win matches. Usmash is a good friend. Up-angled Fsmash is also very nice (amazing once you get his Fair rhythm down).