How do I make a armor wearable?

Supposedly you want to work with Welding or Constraints. Personally for just armour I'd use welding.

You can read up on it here.

I suggest you refer to looking at C0 too, but for that you'd also need to learn CFrame, which can be read up on here.

An example being:

local Weld = Instance.new('Weld')
Weld.Parent = [Preferably put this with Part0]
Weld.Part0 = [Part You Want To Weld]
Weld.Part1 = [Part You Are Welding To]
Weld.C0 = CFrame.new(0,0,0) * CFrame.Angles(math.rad(0), math.rad(0), math.rad(0)) -- math.rad makes it a lot easier to rotate, CFrame.new is for positioning through X,Y and Z.