How do I lock the players mouse in the center of their screen?

You don't need to code that much. Go to StarterPlayer in the Explorer, click on it, in the Properties scroll down to CameraMode and change it to LockFirstPerson. Write this inside a LocalScript then put it in the StarterCharacterScripts inside the StarterPlayer:

local Players = game:GetService("Players")
local mouse = Players.LocalPlayer:GetMouse()
mouse.Icon = "rbxassetid://3504534052"

Change the ID (3504534052) to the Decal's ID that you want to make as the cursor. If you uploaded a Decal and you had changed the ID to your's, but when you're playing it's invisible or says failed to load, wait a few days, and try again.

Remember to use google as well

-------------
--- INDEX ---
-------------

--Services
local userInputService = game:GetService("UserInputService")
local runService = game:GetService("RunService")

--Player & Character
local localPlayer = game:GetService("Players").LocalPlayer

--Other
local Camera = game:GetService("Workspace").CurrentCamera

--------------
--- VALUES ---
--------------

local xAngle = 0
local yAngle = 0
local cameraPos = Vector3.new(3.5,0,7.5)

----------------------
--- INITIALIZATION ---
----------------------

wait(1)
Camera.CameraType = Enum.CameraType.Scriptable
userInputService.MouseBehavior = Enum.MouseBehavior.LockCenter

-----------------
--- FUNCTIONS ---
-----------------

userInputService.InputChanged:Connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseMovement then
        xAngle = xAngle-input.Delta.x*0.4
        --Clamp the vertical axis so it doesn't go upside down or glitch.
        yAngle = math.clamp(yAngle-input.Delta.y*0.4,-80,80)
    end
end)

runService.RenderStepped:Connect(function()
    local Character = localPlayer.Character
    local rootPart = Character:FindFirstChild("HumanoidRootPart")
    if Character and rootPart then
        --Set rotated start cframe inside head
        local startCFrame = CFrame.new((rootPart.CFrame.p + Vector3.new(0,2,0)))*CFrame.Angles(0, math.rad(xAngle), 0)*CFrame.Angles(math.rad(yAngle), 0, 0)

        --Set camera focus and cframe
        local cameraCFrame = startCFrame + startCFrame:vectorToWorldSpace(Vector3.new(cameraPos.X,cameraPos.Y,cameraPos.Z))
        local cameraFocus = startCFrame + startCFrame:vectorToWorldSpace(Vector3.new(cameraPos.X,cameraPos.Y,-50000))

        Camera.CFrame = CFrame.new(cameraCFrame.p,cameraFocus.p)
    end
end)

I'm making an fps so here is the deploy 4 me

game.StarterGui.ScreenGui.Frame.DeployButton.MouseButton1Click:Connect(function()
game.StarterPlayer.CameraMode.LockFirstPerson = true

game.StarterGui.ScreenGui.Frame.DeployButton.MouseButton1Click:Connect(function()
game:GetService("Players")
local Mouse = Players.LocalPlayer:GetMouse()
Mouse.Icon = "www.roblox.com/asset?id=316279305"

I will test this out, if it works better

StarterPlayer MaxZoomDistance will be 0.5