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)

We can use UserInputService and make it so when you hold shift down it sets your walkspeed to 20, and when it is released it is set back to 16.