If you want to save yourself a lot of frustration with the DRebel and flash exposure, set your camera meter to M, adjust your shutter speed to 1/60-1/100 and set your aperture to around f/5.6-f/8 (don't worry that you're not metered correctly for the ambient light...trust ETTL), turn your flash on (are you using external flash? --I never use the onboard flash, so I'm not sure about this), and take the picture. ETTL will automatically provide light based on its own sensor independently of the cameras metering. Experiment with this mode and see what works best for you. I have found that any of the automatic modes do not do well for indoor flash. Outdoors, Av mode works well for fill flash.
Canon's ETTL is different from other manufacturer's TTL flash systems. There is a paper out that explains it in detail and is quite thorough, albeit long and sometimes difficult to understand at first read. http://photonotes.org/articles/eos-flash/