Here is a simple technique to make planet rings, done in Blender 2.64:
This tutorial will assume you already have at least some basic knowledge to how Blender works and that you know how to use the interface and such.
First start a new scene, then add a plane (Add>Mesh>Plane).
Now go into the material settings and click the "New" button.
Now go into the texture settings (one button to the right of the material settings) and click "New", much like you did on the material settings.
Next set the type to "Blend", progression to "Spherical" and under the "Influence" settings, turn on "Alpha".
Now in the "Colors" section (in the texture settings as well), turn on "Ramp", click the "Add" button to add a new point to the gradient and using the button circled in blue, set the color to black and the alpha to 0 on this new point, then slide the points around using the bar (not circled) to match the image to the right.
Then go into the material settings, and turn on "Receive Transparent" under the "Shadow" panel.
This is so that the transparent areas of the plane do not cast shadow over your planet, just the ring.
Also do this to anything that is going to be in the planes shadow.
What you'll want to do next is go back to the "Colors" section of the texture settings (If the settings don't show the first time you click them, click the material settings, then the texture settings) of your plane (ring) and add some points, adjusting the colors, alpha and positions as you've done before in step 6 until you get something you like:
Thanks for reading!