Head Pieces

Spartan (10)
Mark VI.....Avaiblie at start
CQB...........Availible at start
EVA.................Complete Campaign on Normal, Heroic, or Legendary
EOD.................Complete Campaign on Legendary
Security...........Earn 1,000 Gamerscore Points
Scout................Unlock "Used Car Salesman" Achievement (Destroy an enemy vehicle containing 3 enemies in Campaign or ranked playlists)
Mark V.............Unlock "UNSC Spartan" Achievement (Reach Sergeant ranking online)
ODST...............Unlock "Spartan Graduate" Achievement (Earn 5 EXP or complete 10 ranked matches)
Rogue.............Unlock "Spartan Officer" Achievement (Advance to the Spartan Officer ranks online)
Hayabsa......All 13 skulls found
Elite
Flight...............Complete Campaign on Heroic or Legendary
Ascetic.............Unlock "Steppin' Razor" Achievement (Get one triple kill with an energy sword in ranked FFA)
Commando....Unlock "Overkill" Achievement (Get one overkill)

Shoulder Pieces

Spartan (7)
Mark VI.....Avaiblie at start
CQB...........Availible at start
EVA.................Complete The Ark on Normal, Heroic, or Legendary
EOD................Complete The Ark on Legendary
Security..........Earn 750 Gamerscore Points
Scout...............Unlock "We're in for some Chop" Achievement (Destroy an enemy vehicle with a piece of equipment in a ranked playlist or campaign)
Hayabusa......9 skulls found
Elite
Flight...............Complete The Ark on Heroic or Legendary
Ascetic............Unlock "Overkill" Achievement (Get one overkill)
Commando....Unlock "Killing Frenzy" Achievement (Get one killing frenzy)

Body Pieces

Spartan (6)
Mark VI.....Avaiblie at start
CQB...........Availible at start
EVA.................Complete Tsavo Highway on Normal, Heroic, or Legendary
EOD.................Complete Tsavo Highway on Legendary
Scout...............Unlock "Too Close to the Sun" Achievement (Destroy an enemy banshee with the Sp'laser or missile pod in a ranked playlist or campaign)
Hayabusa.....5 skulls found
Elite
Flight...............Complete Tsavo Highway on Heroic or Legendary
Ascetic............Unlock "Up Close and Personal" Achievement (Kill 5 enemies via melee or assassination in a ranked FFA playlist)
Commando....Unlock "Triple Kill" Achievement (Get one triple kill)


Comes from the Official Strategy guide.