As far as I know you can't, but you can turn into Luigi!
Watch the video that the related link goes to. I didn't make the video.
You can sort of, you have to be super guide and whilst you're on super guide click pause and play now, you are then playing as luigi super guide!
New Super Mario Bros.- You hold L and R. While holding, press Start. Mario should transform into Luigi.
you can't unless you playing 2 player
If you have 2 controllers, you can select the second player option on the title screen and once the first player has died it will go to the other person's turn (or other controller) and they will be playing Luigi.
you can only do that after you finish the main game: on the menu where you select your save game press R+L and then the save game you want to play, and TADA! you play as Luigi! have fun!
At the "select a file" screen hold down L+R then press A on any file. Sorry, I didn't see the word "Wii".
No but I have the DS version of the game.
You can't be first player because it makes you be mario, but if you are second, third, or fourth you can choose.
Select a file - L + R + A = Luiginot unless you do ds download with another player/ yes you can At the "Select A File" screen, highlight a file, then hold L + R and press A to play as Luigi in Single Player
Hit 2 player, or you have to play the lost levels