Your Lightroom isn't full. You just don't have any hard disk space. It would seem that you could benefit from adding an external hard drive to your system. Then you could move your images to the external drive to free up some space. How much free space do you have on your hard drive? How big is the hard drive?
I have a My Book WD, but I am not sure how to use it I have formatted it but now I don't know what to do next