Good advice from the others.
If you can, buy a bounce flash. This will make you indoor flash pix much better, if there's a white ceiling.
If not, plan to shoot with the zoom lens at high ISO in RAW format without flash, if it isn't too dark.
If it is dark, then you're stuck with the on camera flash.
When flash shopping, remember that it doesn't have to be a Canon branded flash; just dedicated for Canon. Vivitar, Metz, and Nissin all make Canon dedicated flashes too, and for less money. After reading the reviews, I'm going with a Metz for my Olympus. If you can't afford a bounce flash right now, either rent one or buy a simple, non-bounce off camera flash. Even just getting the flash further away from the lens will make for a huge improvement.