If you're using the Photoshop, the simplest way I've found to create a ring would be to draw a black circle and then work with the Layer Effects (inner glow, outer glow) and play with the "Quality" settings, i.e. curve settings, until you achieve a ring look. Then you can apply some noise, change transparency, hue etc.
I ususally add some rendered asteroids, too, or paint some further details in as well, but that's the basics. I'm sure though that someone has written a detailed tutorial about it. If you search dA for planet rings tutorial it should turn up some various methods!