How to create light graffiti using a Nikon D40?

I have the slightest clue and I just got my camera. I really want to try it out!
Straight forward directions would be great.

Put your camera on a tripod, and use either the self timer or an external shutter release.
Set your camera to a long exposure (say between 15 and 30 seconds, although you might want it much longer than that, depending on your project). Not all little P&S cameras can do this. The shorter time you have, the more limited you are in what you can do with it.
Refer to your manual if you don't know how to do something - you should know that book inside out, anyway (if you lost it, download a new one from the manufacturer).

Make sure you are in a very dark room/area. It helps to have an assistant dressed in dark clothes - less chance of showing up in the shot. Use pretty much any light source you can think of, experimenting with it is fun. Also play with different ISOs and apertures to see the differences there. It will turn out a good learning experience, as well.

So once everything is set up and ready, click your shutter button/timer and have your assistant move around with the light source. Have fun trying to make different designs, all the way from some random squiggle to carefully planned and executed designs.

P.S. Some people call it "painting with light" which is actually a very different technique. Here is an amazing video about it: http://fstoppers.com/night-photography-how-to-paint-with-light/