ok so ive beaten the game right and i chose the sith ending which is where you kill darth vader and i want to get the jedi ending so can i just go to mission select and choose the last mission (death star) and playthrough it then will it still give me the option at the end to kill vader or emperor and will i still get the achievment and jedi costume???? please help because i really dont want to play through the whole game again.

Yeah that works. You could have just played through the last level and checked it that way though....

There is no level select. The game let's you play through the whole game again from the first level to the last. While your at the last you need to save your friend to get the Jedi ending.

quote Kazooieman

There is no level select.

After you beat the game and if you pause the game like the second to last option is level select and you can go to the last level easily.