How to make a health GUI that fills up?

Hello, so I'm trying to understand how GUI's work, but it's giving me somewhat of a headache. I'm trying to make a script that fetches player health & fills for example a stamina icon. How would I go about this? Do I have to make 100 seperate decals?

No you do not have a simple script ready for i have made a few months ago!

wait() 

local Player = game:GetService'Players'.LocalPlayer
local Character = Player.Character or Player.CharacterAdded:wait()
local Human = Character:WaitForChild("Humanoid")

local HealthFrame = script.Parent

Human.HealthChanged:connect(function(Health)
	HealthFrame.Size = UDim2.new(Health/Human.MaxHealth,0,1,0)
end)

make a frame in your GUI and call it Background make the color red and then make a Frame called Positive Then inside it put a local script with the script above also can you show your attempted script maybe more people will help you with it to understand what you are attempting