I'm a Falco main, but the same concept applies for every character on what I'm about to tell you.
The one thing you need to consider is that you're playing against a CPU. In your video I don't really see any tech being applied, unless you're playing like Borp, someone who doesn't rely on advanced tech skill. Also, my initial impression from that video is that you still have a lot to learn. I can't really tell what you need to improve on unless you specify more. Do you want to gain more tech skill like L-cancelling wavedashing, etc., or just stick with regular fundamentals?
One of the biggest tips I can give is for you to research and struggle. Research "Marth" on YouTube and just watch every video that shows someone playing him. It shouldn't matter if it's Melee or PM because the skills learned transfer from one to the other. I'm very knowledgeable when it comes to Falco, but trust me in that there is always something new to learn. Unless you understand how the person did what he/she did, never disregard that video. Make a Chrome/Firefox folder, and just bookmark everything to look at later.
One of the other big tips I can give you is to start using the PC notepad to take notes. Every time you see something interesting or see something that just amazes you, go to your notepad and type it down, then practice it by yourself.
Other than that, I can give you some links to get you started. I'll try to link everything that has to do with Marth, though it doesn't matter with some videos. If you watch one of the videos I linked and don't understand something, research it more on YouTube or make a thread
here.
Wavedashing
https://www.youtube.com/watch?v=jLCX-hSaJpU
https://www.youtube.com/watch?v=N_eZoX8nAdg
https://www.youtube.com/watch?v=m2u51U8j2OA
Dash Dancing
https://www.youtube.com/watch?v=JFLY_GmETxM
SHFFLs and L-cancel
https://www.youtube.com/watch?v=ARN-we6a0p4
Directional Influence
https://www.youtube.com/watch?v=DKMyDkWzOcQ
Tutorials
https://www.youtube.com/watch?v=cmcfXVQvE9E
https://www.youtube.com/watch?v=ZGGHCzl_RMM
Others
https://www.youtube.com/results?search_query=project m marth&search_sort=video_date_uploaded
https://www.youtube.com/watch?v=OkMsQGlcrgU
Players
PM 3.0 &
PM 3.5
For both links, toggle the spoiler for Marth. There's no real major changes to him, so don't worry about his differences in 3.0 and 3.5. If you want to know, click
here and look for Marth's changelist.
If you want some real inspiration, look up PPMD for Melee and Axe for Project M. Axe's PM Marth can be found in the PM 3.0 link.