First off, I am not understanding the fear (scared sh!tle$$ ?)... this should be fun, not scary!!
Second, if you are not sure what to bring, talk to the person that you are going to be coming with, they've done it before and can give you advice too... but...
- fast lenses rule. Club scenes are dark, but have wild wonderful lighting potential. For me, many shots would be done on a lens with F/1.4 apertures (F/1.8 or F/2.8 lenses are about the slowest I would go). As far as focal length, I would take 2 lenses with me... the 24-70 and the 70-200. That is a wide range and covers about any situation that comes up.
- flash can be important. If the ceilings/walls are white, you can bounce it to help avoid red-eye. Off camera light is better, but requires extra knowledge and equipment, though.
- high ISO. Even with the best battery powered flashes, sometimes they are just not strong enough. Raise the ISO to your maximum clean range. This helps the flash not need to work so hard, and in extreme cases, augments the shots you will get with a flash pushing out it's maximum power.
- Enough camera media storage to take at least a thousand pics. Now, you may only take fifty shots, but I would rather have the media for a thousand and not need it, rather than have the media for fifty and need a thousand.
- Have at least 2 sets of batteries for everything that you will be using (ie: 2 camera batteries, 2 sets of flash batteries, etc...)