How to force player to hold items?

Hello ChainMailZombie, you can disable the player's backpack using the StarterGui class. This will disable the player from using the default backpack system.

void SetCoreGuiEnabled (
    CoreGuiType coreGuiType,
    bool enabled
)

You can then call the SetCoreGuiEnabled function from the StarterGui object to disable the player's default backpack.

-- Grab reference to StarterGui so you can use the class from the Roblox's API
local StarterGui = game:GetService("StarterGui")

-- Disable the default backpack system
StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Backpack, false)

From there you can use the UserInputService that Roblox provides on there API to check for input events on the keyboard such as InputBegan or InputEnded.

It could get really complex or you can make it pretty basic with just keyboard input to change the player's current Tool. If you have more questions I would gladly help. I just gave you some hints on how to start your task.