Instructions
Play the "Campaign" mode -- each level you complete rewards you with credits. The higher the difficulty, the more credits you earn as well. Additionally, completing single player challenges, such as "Kill 100 Grunts" earns you additional credits. These challenges can be viewed from the "Pause" menu at any time.
Complete games in the "Multiplayer" game mode. For each game you complete, win or lose, you are awarded bonus credits, with more credits given if you win. Additionally, killing other players and completing objectives in multiplayer earns credits.
Earn medals in multiplayer to gain additional credits for each medal earned. You can earn medals by performing specific tasks in the game. For example, killing three enemies in rapid succession earns you the "Triple Kill" medal. For each medal you earn, you are given additional credits once the match is over.
Play the "Firefight" game mode. "Firefight" pits you and up to three friends against waves of enemy soldiers on an enclosed map. You are awarded credits based on how long you survive and how many enemies you kill.
View your rank and the amount of credits you have earned by selecting "Armory" or "Player" on the "Halo: Reach" main menu.