How do I make a health bar?

Simple division

The primary purpose of a health bar is to display a percentage indirectly with a GUI. All we have to do to get the percentage of something, is divide the difference by the initial value. In this case, the player's current health divided by their max health.

Explanation

I'd explain this more in depth, but I've actually created a YouTube video on this if you're interested. I go over the basis of creating and coding a custom health bar, along with providing the source code. You can find the video here: https://www.youtube.com/watch?v=opZtUhwDWE8

Making the structure of the health bar can be self-explanatory.

However, this is the code:

local player = game.Players.LocalPlayer
local fill = script.Parent.Filler
local text = script.Parent.TextLabel
while wait() do
    local character = player.Character
    if character ~= nil then
        local human = character:FindFirstChild("Humanoid")
        if human ~= nil then
            fill:TweenSize(UDim2.new((human.Health/human.MaxHealth),0,1,0))
            text.Text = "HP: "..math.floor(human.Health).." / "..human.MaxHealth.." ["..math.floor((human.Health/human.MaxHealth)*100).."%]"
        end
    else
        text.Text = "NIL"
        fill:TweenSize(UDim2.new(0,0,1,0))
    end
end