Shooting moon with nikon d3100?

What settings to use

Manual, by doing this you can learn to better understand your camera.

Try this - all the help you could ever hope for: http://lmgtfy.com/?q=moon+photo+tutorials

Use lowest F number! (like 5.6)
Set Shutter to highest number that will be exposed well!
Set ISO by experience!

Zoom in and Frame the Moon in the middle (it get's Sharper)
Then Crop in Computer!

I think you could have figured this out yourself had you thought about it.

If you look through your longest lens, you will see that the longer the lens the better.

If you think about the light falling on the moon, you will realize that the same sun illuminates the moon as the earth, the the Sunny 16 rule applies. 1/ISO @ f/16

Lens: on a 1.5x crop sensor camera like yours, at least 300mm. 500 to 1000mm is better. 200mm is marginal. The moon is only 0.5° across.

Exposure: The moon is very bright. ISO 200, f/8, 1/160 shutter speed is a good starting point. You may need to fine tune these numbers. Enable highlights review mode to catch overexposure, or use the histogram. If you didn't figure this out yet, you need to shoot in manual mode. Be sure auto ISO mode is disabled.

Set white balance to daylight.

Shoot raw. You'll need to do post processing to bring out the details by adjusting levels. It's best to work on a 16-bit image when you do this sort of manipulation.

Turn off sharpening in the camera. The high contrast edge will look very bad if you have sharpening turned on. In post processing, you can sharpen (use the unsharp mask) the image a bit, but watch what happens at the edges.

Use a tripod.