u'll find that there are sooooooooooo many threads like this. "how do i beat my friend? he's good but im better so how am i losing?" the answer could be ANYWHERE. ur spacing could be off, u might be using laggy ground moves too much, you might not be edgeguarding enough, u might be falling for SHL spamming, you might not know what part of the stage to keep to, you might be wavedashing for no good reason and not using it sensibly, your shffling might be pointless or predictable, you might not be using marth's side-B recovery.
If any of these accusations sound absurd, it's because you havn't provided enough info about how you play. For all we know you could be the nubiest of the newbs, or you could be Ken's reincarnation with a brother who's PC chris's. WE NEED VIDEOS