Theres actually 121 Stars in the game, not 120. To get it, you must first have all 120 Stars. This unlocks Luigi. Beat the entire game with Luigi, defeating Bowser at the end. After this, a new galaxy will be unlocked. Finish this final galaxy to get the 121st Star.

Heres how to get Luigi. First, get all 120 Stars in the game. Then, return to Rosalina (on the Comet Observatory) and request to fight Bowser one last time. Defeat him and let the credits roll. Thereafter, a message will appear telling you Luigi is a playable character.

The Grand Finale Galaxy, which is the secret galaxy that can be unlocked after 120 Stars are gathered with both Mario and Luigi, holds another secret. Once the final
Star is acquired with both characters in this galaxy, you will be able to see your death count on your save file. This will allow you to see how many times you died while playing the game with both Mario and Luigi.

If you have a second Wii controller, you can have a friend play along with you. While there wont be a second player on-screen, you will have a second cursor to use. This means that Coins, Star Bits, and the like can be collected by the second player. Theres also some functionality in controlling Marios jumps.

Because you start each game with only four lives (after shutting the game off and turning it back on), you might be in need of some extra lives upon rebooting the game. You can usually get five right off the bat from Toad (hell run around with a letter above his head, symbolizing a letter from either Luigi, or more likely Peach). There are four more strewn around the Observatory that you can acquire, however: