ROBLOX UserInputService

local UIS = game:GetService("UserInputService") --Instead of "UIS" you could also say "UserInputService"

UIS.InputBegan:Connect(function(key)
    if key.KeyCode == Enum.KeyCode.F then --You could use any other key on the keyboard other than "F"
        print("F") -- It doesn't have to be a print. You could make it do whatever!
    end
end)

-- This is just an example of UserInputService. Don't use this exact code for an actual game, unless your just using "print("")" to test that it works! :D