If you want to see some of the flashiest Marth play (or at least, really flashy), look up "Messiah - a Marth combo video" [by Mr. R if I'm not mistaken]. Since that stuff is done on actual opponents, it would mean that you could theoretically use it on others (DI-dependent and stuff).
If you don't know it, make sure you learn basics, like DI, airdodge lengths and how Marth should punish them, SDI, momentum cancelling and Marth's most efficient way to do so (And make sure you have a feel for when you actually need to use your jump to momentum cancel), KO percents for Marth's KO moves on various characters and not staling those moves early (MK, Diddy Kong, and Snake are three good characters to know rough numbers for - lightweight, middleweight, and heavyweight) and more (I don't know what you already know and don't know, so I'll just stop listing things - if you don't any of these things they are good things to learn about for starters).
Also, learn to fight MK. He's Marth's hardest MU, or at least the hard MU you'll likely fight most frequently (Dedede is also not easy to deal with, since he has a chaingrab on Marth, among other reasons).
For reading on basic things, this is a good place to start (the Marth boards - has MU discussions and a Q&A forum, and I believe a thread that links to various Marth guides):
http://smashboards.com/forums/marth.168/