How do I add a sprinting feature?

Here you go:

Create a LocalScript inside of StarterCharacterScripts located under StarterPlayer and insert the script below:

local plr = game:GetService('Players').LocalPlayer
local humanoid = plr.Character:WaitForChild('Humanoid')
local UIS = game:GetService('UserInputService')
local defaultSpeed = humanoid.WalkSpeed --[[ Gets the default walkspeed]]

UIS.InputBegan:Connect(function(input) --[[ Runs when any input is detected]]
	if input.KeyCode == Enum.KeyCode.LeftShift then --[[ If the input is the left shift key]]
		humanoid.WalkSpeed += 16 --[[ Increases WalkSpeed. Change this to your needs.]]
	end
end)

UIS.InputEnded:Connect(function(input) --[[ Runs when any input is ended (Like a keyUp thing)]]
	if input.KeyCode == Enum.KeyCode.LeftShift then --[[ Again detects if it's the left shift key]]
		humanoid.WalkSpeed = defaultSpeed --[[ Sets your speed back to the default WalkSpeed (16 by default whenever you create a game)]]
	end
end)