how do you unlock the cool new armors in halo 3?

[deborah q]

u start out with a regular sparten outfit and an elite one too. but i no u can get more

 

[High 'N' Dry]

By unlocking various achievements. While playing the campaign, do not play it on easy, you won't get any achievements that way. Playing it on normal, heroic, and legendary will, however. By doing various things while playing the game will unlock even more achievements. Also collecting skulls will get you achievements and some very cool armor. Here is a list of all the achievements in Halo 3 and the way to get them. The site also has information about where all of the skulls are located, and hints and tips for the game:
http://www.gamefaqs.com/console/xbox360/code/926632.html