how would i detect if i took damage?

You would need to use :GetPropertyChangedSignal()

local health = 100
player.Character.Humanoid:GetPropertyChangedSignal("Health"):Connect(function()
    if player.Character.Humanoid.Health < health then
        -- code goes here
    end
    health = player.Character.Humanoid.Health
end)

This will only fire if the humanoid health is LOWERED (aka damaged).

You could also do

Humanoid.HealthChanged:Connect(function()
-- code
end

this way you can fire a function whether the health goes up or down