First thing I thought of when I read your description of the ambient lighting was white-balance issues as Trevor mentions. Downlights, fluoro lights, and flash all emit different coloured light. Definitely shoot RAW if you aren't already. It will be infinitely easier to correct later on.
Good tips above - bounce the flash off the ceiling. Makes the ceiling a huge light source so softer light. Be aware that you might get shadows under people's eyes etc because the light is coming from above them. Look up "bounce cards". Just get a sheet of white paper/cardboard (maybe A5 sized) and a rubber band. The idea is that most of the flash light will bounce off the ceiling, but the bounce card will direct some light forward to give a bit of fill to lift some of those facial shadows. Think about the paths light is taking.
Not sure how big the room is or how many people. If it's a big function, be aware that you may not have enough flash power to light the whole room. Backgrounds may be extremely dark or black.
Exposure-wise, if you're relying on flash as your predominant light-source, shutter speed is not really import as a general rule. The flash burst will be much quicker than your shutter speed. It's all about aperture and ISO. Robert's recommended settings above are the ballpark I was going to suggest. Notice he' recommending pretty wide apertures and a pretty high ISO. This will make your flash work less hard, so batteries will last longer and recycle times quicker. Also it may help ambient light in a bit more so all of your backgrounds are less dark.
If you can get there before the event, practise some shots with noone there. If not, practise at home. A lot. Become comfortable with the flash and adjusting the settings.
Oh, and have fun. Don't let the photography consume your night.
