Since you are in first person, theres a user input for camera movement.
Enum.UserInputType.MouseMovement
to tell me the delta x/y i would just do this
local context = game:GetService("ContextActionService")
context:BindAction("CameraMovement", function(_,_,input)
print("x is: "..tostring(input.Delta.X), "y is: "..tostring(input.Delta.Y))
end, false, Enum.UserInputType.MouseMovement)
(Untested but should work)