"create a movement system where you can slide (by holding c and running, sliding's speed decays really fast and eventually stops you from sliding), crouch (by holding c), run (by pressing shift), walk and have an idle animation where do i put the code"

-- local script 
local UserInputService = game:GetService("UserInputService")
local player = game.Players.LocalPlayer
local character = player.Character
local humanoid = character:FindFirstChild("Humanoid")

local running = false
local crouching = false
local sliding = false

local function onKeyPress(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        running = not running
        if running then
            humanoid.WalkSpeed = 24
        else
            humanoid.WalkSpeed = 16
        end
    elseif input.KeyCode == Enum.KeyCode.C then
        crouching = not crouching
        if crouching then
            humanoid.WalkSpeed = 8
            humanoid.JumpPower = 0
        else
            humanoid.WalkSpeed = 16
            humanoid.JumpPower = 50
        end
    end
end

local function onKeyRelease(input)
    if input.KeyCode == Enum.KeyCode.C and running then
        sliding = true
        humanoid.WalkSpeed = 50
        wait(0.5)
        while humanoid.WalkSpeed > 0 do
            humanoid.WalkSpeed = humanoid.WalkSpeed - 2
            wait()
        end
        sliding = false
    end
end

UserInputService.InputBegan:Connect(onKeyPress)
UserInputService.InputEnded:Connect(onKeyRelease)

-- You can put this code in a LocalScript inside StarterPlayer > StarterPlayerScripts. Make sure to have a Humanoid in your player's character model for this to work. You might also need to adjust the walkspeed values to your liking.