There are three factors that determine exposure - sensitivity (ISO number), how long the shutter is open (shutter speed), and how wide the lens opening is (aperture). You want to optimize all three for your shots.
You want to use the lowest ISO possible. The higher the ISO, the noisier the picture. From the reviews I've seen, the Rebel XTi is quite good to ISO 400. At 800 you start to see a lot of noise. At 1600, you see a tremendous amount of noise. You can eliminate some noise later with special tools like Neat Image or Noise Ninja, but doing so also eliminates fine detail. In general, try to use ISO 400 or below and go higher when you need to in order to get the shutter speed and aperture combinations that you desire. The only exception would be when you want a noisy looking shot for artistic reasons.
In general, you want to use the highest shutter speed possible. That reduces blurring because of camera shake or subject motion. The traditional rule of thumb is to make sure that your shutter speed is at least as fast as the reciprocal of your focal length. So for a 50mm lens, you would want a shutter speed as fast or faster than 1/50 of a second. That's a good starting place, but you'll want to experiment with your lens at different shutter speeds to see what gives you acceptable results. Using a tripod allows you to reduce the effect of camera shake even more. You still have to worry about subject motion. As a general rule, I would recommend that you not shoot below 1/60 with that lens unless you have to or you are trying for some creative effect like a panning shot.
There isn't a general rule for aperture settings. The wider you open the aperture (lower f-stop number) the more light you let in and the shallower your depth-of-field. The narrower you open the aperture (higher f-stop number) the less light you let in and deeper your depth-of-field.
Depth-of-field refers to the range in the photograph where things appear to be in focus. If you want your subject to look sharp against a blurry background, you use a wide aperture. If you want everything in your picture to look sharp, you use a narrow aperture. Just be aware that as you start using apertures narrower (f-stop higher) than about f11, you begin to lose overall sharpness do to diffraction effects. Most lenses are sharpest at about f8.
When shooting in low light, pick the aperture that gets you the shutter speed that you want, but be aware that a wider aperture will reduce the depth-of-field. That may be good or bad depending upon the look you want. Also be aware that narrow depth-of-field makes accurate focusing more critical.
If you switch to full-auto mode, the camera will try to pick the best settings for you. It actually does a really good job, except that it has no idea what you want in terms of depth-of-field. If you go to a manual mode, make it either P, AV, or TV and not M. M requires that you set both the aperture and the shutter speed yourself and that you make sure that the combination results in a good exposure. AV and TV allow you to pick the aperture (AV) or the shutter speed (TV) and the camera will pick the other to match. P tells the camera to pick both (like full-auto) except that it still lets you overrule the camera and make adjustments on the fly.
I would recommend experimenting before you take your trip. Try taking shots in different rooms of your house at different light levels. See what the effects of different apertures and shutters speeds are. That will give you a better sense of what to expect.
Another thing that you might want to consider is shooting in RAW instead of JPG for your trip. It's more hassle to "develop" the pictures, but it gives you more lattitude if you botch your exposure level.
