I want a loop to run for as long as a button is pressed down, but I have no clue how, help pls?

I honestly am clueless, I've tried something by the likes of this:

function decreaseStamina()
	while running == true do
		Stamina = Stamina - 1
		print(Stamina)
		wait(.01)
		
	end
end

but upon key release the loop would keep going.

I suggest UserInputService (LocalScript):

game:GetService("UserInputService").InputBegan:Connect(function(input)
	if input.KeyCode = Enum.KeyCode.LeftShift then -- Replace LeftShift with any KeyCode
		Stamina = Stamina - 1
		print(Stamina)
		wait(0.01)
	end
end)

UserInputService and KeyCodes

Edit: Wrote "Conenct" instead of "Connect"

-- Put in StarterCharacterScript as a LocalScript

local player = game.Players.LocalPlayer
local character = player.Character 
local humanoid = character:WaitForChild("Humanoid")
local input = game:GetService('UserInputService')

local reloading = false
local max_walkspeed = 32
local min_walkspeed = 16
local max_stamina = 30
local running = Instance.new("BoolValue") -- So we can use the Changed event
local stamina =  Instance.new("NumberValue")
stamina.Value = max_stamina



local function change_walkspeed(ws)
	humanoid.WalkSpeed = ws
end

local last_stamina = stamina.Value
stamina.Changed:connect(function()
	print( (stamina.Value < last_stamina and 'Stamina Depleting: ' or 'Stamina Rising: ') ..stamina.Value )
	last_stamina = stamina.Value
end)

running.Changed:connect(function()
	if running.Value then 
		change_walkspeed(max_walkspeed)
	else
		change_walkspeed(min_walkspeed)
	end
end)

input.InputBegan:connect(function(inputType)
	if inputType.KeyCode == Enum.KeyCode.LeftShift and not reloading then 
		running.Value = true
		while running.Value and stamina.Value > 0 do
			stamina.Value = stamina.Value - 1
			wait()
		end
		-- while loop breaks and stops us from running
		running.Value = false
		if stamina.Value <= 0 then 
			reloading = true
		end
	end
end)

input.InputEnded:connect(function(inputType)
	if inputType.KeyCode == Enum.KeyCode.LeftShift then 
		-- If we let shift go and we're running, stop us. 
		running.Value = false
	end
end)

while true do 
	if reloading then 
		-- if stamina is full
		if stamina.Value >= max_stamina then 
			reloading = false
		else
			-- else deplete
			stamina.Value = stamina.Value + .5
		end
	end
	wait()
end