Yeah- what Killock said
While I wouldn't suggest looking to Leon for how to play a MU, he plays the game incredibly well. He's ridiculously smart and has amazing *fsmash* spacing.
Ramin, MikeHaze, and MikeNeko are the three best Marths in the world, and they each excel at different things.
The best thing you can do, Arxis, is to watch MikeNeko, Raz, Nike, Killock, NEO (lots of NEO), Ramin, Leon, Kadaj, Pierce, MikeHaze, and Tyrant use Marth. Different players excel at different aspects. When watching them, the most important question you can ever ask yourself is "why?" Why did NEO choose to grab here when he used fair in the past? Why did MikeNeko recover low here when MikeHaze recovered high; did one work better than the other; if so, why? And so on and so forth.
You also want to make sure you develop your own game. If you just add things from other people's playstyles into yours, without fulling understanding why they choose those options, the addition of these things will be a hindrance more than a boon.
There are a LOT of guides here on the Marth boards that you should look over frequently and thoroughly, here's a few of the more basic ones:
http://www.smashboards.com/showthread.php?t=183969
http://www.smashboards.com/showthread.php?t=315099
http://www.smashboards.com/showthread.php?t=263946
http://www.smashboards.com/showthread.php?t=164063
http://www.smashboards.com/showthread.php?t=235157
http://www.smashboards.com/showthread.php?t=206347
http://www.smashboards.com/showthread.php?t=229253
http://www.smashboards.com/showthread.php?t=165861
http://www.smashboards.com/showthread.php?t=317800
Not EVERYTHING in each of the threads is completely accurate all the time (many of them are quite old, please don't necro them if they are) but they're a good place to start. Pierce, Kadaj (when he gets on), Ramin, and myself are the best people to ask questions if you have them since we're the most likely to give you complete and accurate answers (unless you annoy me of course =p ). The Q&A thread typically will get your answer fairly quickly and the social thread is fairly active. If you have videos of yourself, that are not wifi, post them in the appropriate subforum and someone will get to them. Please make sure that, before posting the matches, you read through my stickied thread in the subforum first.