People in bars or restaurants is one of the toughest lighting situations for you to master. First, there is low light and often comes straight from the ceiling as spot lights. Second, people are moving, and that makes it even tougher to apply techniques that are ok for static scenes, like a tripod - a tripod does not help with subject motion. Finally, your equipment is probably not made out of very fast lenses or high ISO capable sensors, so you're limited from that point of view as well.
Flash could be the answer for you, but you have to be aware that the color of flash light is very likely different from the ambient light in the bar. Unless you match the color temperatures you'll get a typical "flash effect" of snapshots.
My suggestion would be to use these four techniques:
(1) Set the camera to high ISO - 800 or so as I am not sure the noise your sensor will create above 800 may be acceptable, but you need to experiment.
(2) Gel the flash to map the ambient color - normally a tungsten color balancing gel (Color Temperature Orange or CTO) gel will do. You can buy a gel kit for about $10 here -
The Strobist Collection by Rosco
(3) Bounce the flash - you may be able to do that with a piece of white cardboard attached to the flash, with the flash pointing up - this will soften and diffuse the light, at the expense of reducing its efficiency. You may also want to experiment with flash compensation to avoid an obvious flash effect. Try -0.6 or -1.0. Use rear flash mode.
(4) Drag the shutter - use a low shutter speed to try to capture as much of the ambient light and the "feel" for the place. Try 1/30 sec or so.
If you experiment with these three approaches, setting the ISO high, letting the camera compute the exposure for you using TTL, drag the shutter and use a gelled diffused flash, you may get acceptable results.
Post some images!