
Fort Northreach Storage Key for game's first locked door is easiest one to find.
Key
Follow the main pathway tutorial showing and you will see the key with a shield on the ground in the next room from the locked door.
Reward
Unlock the door and loot the chest in front of you for the Common Breastplate.
- Common Breastplate has better damage reduction than you current chest item, so it's better to use it.