This can be done the first time you ever put the disk in to your console. When you load up the game, you will be asked to enter a few details and you will be given the option to customise your spartan. Every new player starts with enough credits to buy the most cheapest item. Simply purchase an armor part from the armory and there you go, achievement unlocked. To get to the armory any time after you first load up, simply press start when at the main menu and then select "Armory" from the list that appears.