Ok, it looks like you need quite a bit of work. I'll just name some things i found while watching the match.
0:13 The first thing you do is a dash attack. Dash attack is one of Marth's worst attacks. Your better off with a SH Fair in pretty much any scenario.
0:15 You attempt to roll behind him, bad idea. And even worse you ended up rolling directly in front of him. Directly into a FSmash...
0:16 You DSmash right away, bad idea. If there's one thing you need to take away from my comments, its that you need to smash less. Smashing is only good if you are certain you will hit, or your opponent is in kill range. Their are easier and safer ways of racking up damage.
0:23 The dolphin slash their was extremely risky, and you were punished for it
0:25 Again, stop smashing
0:27 Dair on the stage is pretty much never a good idea, easily punishable because of landing lag. You should have just used fair
0:30 You just randomly attack like 4 times in a row. 95 % of the time you'll be punished for that.
0:44 You jumped over his head and started attacking, not a good idea. he got in a lot of damage for that
I'm not gonna do this for the entire vid, so let me just sum it up:
It looks like you just need to play a lot more and get the fundamentals down. Most importantly spacing and move choice. The biggest thing I noticed is that you play WAAAAAYYYYY too aggressively. You look like a berserker out there. Marth, as most characters in this game, should be played defensively. Shield more, spot dodge, etc. Don't ever roll behind your opponent though, I saw you do that a few times, it's basically handing over free damage.
Hope that helped, keep playing and improving. Post another vid once you learn more and get better.