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.