How To Unlock Characters(acording to other sites)

clones:
Dr. Mario:beat the game as Mario on classic mode on very easy without losing a life

Gannondorf:beat event 29

Falco:beat 100 man spar

Young Link:beat the game as Link on classic mode on very easy without losing a life

Pichu:unlock Luigi and Falco, then beat event 37

Roy:beat the game as Marth on classic mode on very easy without losing a life

characters:
Luigi:beat stage one of Adventure mode with the last digit of seconds as a 2, you'll fight Luigi instead of Mario, beat him in less than 1:00 and continue and beat Adventure to unlock him, or play a certain amount of matches

Jigglypuff:beat classic mode

Mewtwo:play 700 matches or play for 20 hours(whichever comes first)

Marth:play a certain amount of matches

Mr. Game & Watch:beat the game on classic with all other 24 characters, or beat break the targets with the other 24 characters

How To Unlock Stages(acording to other sites)

Game and Watch level: beat the game as Mr. G&W on very easy without losing a life

Super Mario 2(USA):beat the game with Peach on very easy without losing a life

Brinstar Depths:play 50 matches

fourside:play 100 matches

Big Blue:play 150 matches

Pokefloat:play 200 matches

Dreamland:beat Break the targets with all 25 characters

Yoshi's story:get 400 meters with Yoshi on the snadbag minigame

congo Jungle:beat 15 minute spar.

battlefield:beat all star mode

last level:beat event 51

Miscellaneous Stuff(acording to other sites)

all star mode:unlcok all characters

sound test:beat event 51

random stage select:unlock all stages

events 31-50:beat 1-30, unlcok Luigi, Dr. Mario, Young Link, Falco, and Jigglypuff

event 51:b eat events 31-50

Pikmin trophy:have a Pikmin file on the same Memory card as SSBM