I've never been happy with the results of the built-in flash in a party environment, because it tends to wash out the ambient light and the subjects have an unnatural skin color. If you use it, try to diffuse the light by taping a piece of paper over it or something. If you don't have an off-camera flash you can use, my recommendation would be to with with the fastest prime you've got, and the wider, the better. Anything above a 50mm focal length will just frame your subject with little context of the party around them. Even 50 is a bit too close in a crowded party. Best lens would be a 24mm or 35mm at f1.4 or better. Actually, regardless of the flash, you should try to use a very fast, wide prime.