How to get the best out of my camera taking club pictures?

A great friend of mine is having a party at a venue and I'm sure there will be little lighting and lots of dancing. I'm only a beginner so I'll be doing this for free.

I have a Nikon D3100 with internal flash only VR 18-55mm 1:35-5.6G

The club shooters I know use fast lenses like the 24-70 mm f/2.8 on cameras like the Nikon D300s and D3s bodies or Canon 7D or 1Ds, Mark III. As you can see, shooting in low light really requires a substantial investment

It's going to be very dark and you do not have a lens with a larger aperture, so using no flash is out of the question.

I'd say just have fun with it, use your flash in formal photos of people, and when people get up and dancing, try out the slow-sync flash on your camera. Look it up in the manual if you have trouble turning it on.

Basically, a slow-sync flash allows you to use the flash with slow shutter speeds. With a shutter speed of, say, 2 seconds, you can move/swing the camera around a bit then point it at your friends. It will flash at the end of the 2 seconds, and the flash will freeze the motion of your friends, not to mention the photo will have all the shaky light effects you just did.

Here's an example: