You can customize your appearance by mixing and matching the Halo 3 Armors. However, before you can do so, you will have to earn various achievements before you can unlock the armors.

The most coveted armor is the Katana armor, which can only be unlocked when you get 1,000G (Gamerscore points). This armor has a long sword at the back.

The Halo 3 Unlocking Armor is as follows:

EOD (Helmet) - Complete the campaign mode (on Legendary)
EOD (Shoulders) - Complete The Ark (on Legendary)
EOD (Body) - Complete Tsavo Highway (on Legendary)

EVA (Helmet) - Complete the campaign mode (on Normal)
EVA (Shoulders) - Complete The Ark (Normal, Heroic or Legendary)
EVA (Body) - Complete Tsavo Highway (Normal, Heroic or Legendary)

Scout (Helmet) - Get the Used Car Salesman Achievement
Scout (Shoulders) - Get the We’re in for Some Chop Achievement
Scout (Body) - Get the Too Close to the Sun Achievement
Security (Helmet) - Get 1000 Gamerscore points
Scout (Shoulders) - Get 750 Gamerscore points.

Hayabusa (Helmet) - Collect 13 skulls
Hayabusa (Shoulders) - Collect 9 skulls
Hayabusa (Body) - Collect 6 skulls

Katana (Complete) - Achieve 1000 Gamerscore points

ODST (Helmet) - Get Graduate Achievement (Online)
Mark V (Helmet) - Get the UNSC Spartan Achievement (Online)
Rogue (Helmet) - Get Spartan Officer Achievement (Online)

EOD = Explosive Ordinance Disposal
EVA = Extra Vehicular Activity