On the menu of the game, press "L" and "R" at the same time to play as Luigi.
No you can play as Mario and Luigi.
You be Luigi in New super Mario bros DS by holding L + R while choosing any save file.
In the original you can play as Mario and Luigi. In New Super Mario Bros DS you can play as MArio AND LUIGI. IN NEW SUPER MARIO BROS WII ITS MARIO, LUIGI, BLUE TOAD, AND YELLOW TOAD.
2. Mario and Luigi - L+R+A (you press the buttons on the ds.)
ds,mario and luigi,wii mario, luigi, blue toad, and yellow toad.
Mario and luigi, to play as luigi, hold L+R while loading a save/profile
Press L and R and A at the file selection screen
You need to defeat Petey Pirhanna as mini Mario or mini Luigi.
As far as I know you can't, but you can turn into Luigi!
On new super Mario you hold down l and r at the same time when you hit the game file
press L+R+A when selecting a file to play (A = to select the file)
if youre talking about the ds game, then the answer is you dont. sorry...