Talent show Photography in dark auditorium?

Okay, so I need help with settings for a Nikon D3100 with an 18-55mm or 55-200mm lens. What would be easiest without a tripod or monopod?

The stage may look dark, but how are the contestants lit? The 18-55 will be the lens to use if you can get close enough. You will need to shoot in manual mode which means you need to know the relationships between aperture, shutter speed, and ISO.

As a starter, you can set your ISO to 1600. Anything higher than 1600 will increase the noise in the image.

Set the aperture wide open (in the f/3.5 to f/5.6 range, depends on focal length used).

Shutter speed, try keep it faster than 1/30 seconds, however, if you are good at holding a camera steady, you can go down to 1/15 seconds with the 18-55mm lens. Adjust your shutter speed to give the correct exposure. If your images are too dark, you'll need to increase the ISO if your shutter speed is slower than 1/30 seconds.

If you use the 55-200mm lens, you'll need to keep the shutter speed higher, perhaps 1/100 seconds. General rule of thumb is the shutter speed should be 1/(focal length), or faster when hand holding.

You'll need to set the white balance to match the lighting. If they are using LED lights, there's little hope that you'll get the color balance right unless the lighting person knows how to deal with mixing colors. If you know how to process raw images, this gives you the opportunity to set the white balance in post processing.

Something that will improve the stability of the camera is bracing your elbows against the arm rests on the seat, or lean forward and use the back rest of the seat in front of you (have your mate sit in front of you so you can annoy them). You can also rest your feet on the seat cushion and use your knees as a brace.

Learn to use the highlight feature on your camera. This will tell you when parts of your image are overexposed by flashing the blown out areas. You want to avoid overexposing your subject.

Practice in your living room. Nothing worse than trying to set up things when under pressure.

If this is going to be a regular occurrence, you should look in to a faster lens, perhaps a 50mm f/1.8 AF-S lens. You can also get 100mm f/2.8 macro lenses which will work pretty good for shooting stage.