If you pre-ordered the game or got the Legendary edition like I did, then you actually got codes for armor, so you could get this achievement when you first start up the game.
But if you didn't pre-order it or get the Legendary Edition, then you get your first armor at SR8.
1) Press the start button.
2) Go to the customize tab.
3) Click Spartan Armor.
4) Click Torso.
5) You should see a new piece of armor called Warrior.
6) Equip this and the achievement should unlock.