And with it, a wonderful opportunity for creative photography. My advice in simple is, broad apertures, carefully selected white balances, and stringent use of flash as a main source of light (though careful fill flash can work well if done right).
How to accomplish this? The easiest way:
Step 1. Get and use a broad aperture prime (50 1.8′s can be had for cheap!). Use it at f1.8 (or whatever your braodest aperture is) as much as possible. Just be careful to keep the eyes in focus. For group shots, stopping down to at least f2.8 is advisable. But, a broad aperture will provide beautiful out-of-focus highlights from the Christmas lights. A nice open aperture will get you more light, decreasing your need for flash…
Step 2. Learn your camera. Figure out how to control white balance. Likely, your indoor photos will be very orange if you skip this step. If you must use flash, figure out how to control your flash output, to keep the flash from overpowering the scene. The best flash is a hotshoe flash which can be bounced off of a ceiling, wall, or something. Lacking this, learn to balance your flash (even a pop-up flash can do this with an slr) with the ambient enviroment. More on this later… But the easiest method is to skip flash and use ambient light and high ISO’s, if need be. Note, I did not say easiest is best; it is simply easiest.