All @Ignis: I've heard that NorCal has a decent amount of tournies, actually. Are you in touch with the scene, or do you kinda just go to what you catch wind about?
As for Marth vs. Snake, it's a pretty difficult matchup unless you have experience. You're going to have to focus on a lot of defensive play, short-tossed grenades, retreating C4, and the like. Marth beats you up close, so you kinda have to bring the momentum to yourself by staying back and using projectiles and stuff as a defensive measure (like I said before). Just feel it out in some friendlies and see what comes naturally. Read some of
this thread if you want some more specific information from some more knowledgeable people.
I like practicing on wifi, honestly. I prefer it to the M2K / Jem approach of setting up situations against CPUs / blanks in Practice mode. Just be aware of the changes between offline and wifi play. Things like the increased lag forcing people to slow their button inputs down, etc.