I want to use my laptop as a monitor for my PC tower. How do I hook it up?

Unless the laptop supports some type of video-in or video-passthrough functionality, you don't. The laptop isn't a monitor, it's a computer with an integrated display device. Now, you could disassemble the laptop and with enough electronics savvy jury-rig the video parts to function as a monitor; but, then it wouldn't be a laptop anymore.