The absolute best ways to learn to use a character(in my opinion) are as follows:
1) Learn the moveset. Look at the M2K guide and go into practice mode to learn the moveset. If you don't, you'll never really become great with any character.
2) Watch a video of a GOOD player with that character. Analyze what works, how they play, and WHY they're good(for marth I would recommend either of the two videos.
http://www.youtube.com/watch?v=QM5X9wFA1LY
http://www.youtube.com/watch?v=fqAzaaIADTc
3) Go into practice mode and fight level 3 computers and try and develop a game plan similar to that of the videos. Take what you like, try and adapt. Get a feel for the character. Remember, they're only level 3's.
4) Here's where you can either try battling a friend online(though offline is preferred), or going to a tournament and getting your *** kicked. Honestly, play against a friend as much as you can first or you'll just get discouraged. Once you feel comfortable with him, go to a tournament and prepare to be open to criticism. Video tape your matches if possible and upload them for critique.
5) Practice....honestly, this one is true for anything in life. Brawl is no exception.
6) Keep up to date on the character forum. The marth forum is actually pretty active. Everyone will tell you to go read emblem lord's threads because he's basically one of the leaders on this forum. Best of luck and never be afraid to ask for assistance. We're usually pretty nice about it =]
P.S. Don't play marth exactly like you did in melee. FAIRS ARE FRIENDS NOT SPAMFOOD.
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