Hi there, it all depends on what you want to shoot at night. There is going to be millions of situations and settings, more or less. First off the tiny flash on your XTi is useless. If you think you require a flash get a real flash like one of the 430 EX models.
Here are some night shots that have very different settings. Each requiring special attention to details possible multi-exposures. They are not snap shots, to say the least.
Besides settings in the camera soem shots require serious editing in post. Can you be a little more specific on what you want to shoot at night? What gear do you have exactly? Besides the flash, a good tripod is usually a must have.
"... everything. fireworks"
Fireworks are among the easiest to shoot. Set the Rebel on a tripod. Manual mode. Av to f8 and SS as long as you want. Shoot away.
You make it sound easier than it really is. Most fireworks pictures don't turn out nearly that well.
the sky... stars... people, buildings.... etc... everything. fireworks
This is how I would try typically focus on that sort of stuff
As far as everything goes, I generally try to keep shutter speed high and ISO low, for sharp pictures. There are good reasons to use a slower shutter, though, because it can used to convey a sense of motion in various ways.
The flash is flickering because it is too dark for your autofocus to see what it needs to focus on and the camera is trying to use little micro flashes to light up the scene enough to autofocus. The busy indicator is just telling you this.
Rebel cameras, especially older ones, really struggle to AF in low light. I had a T3i and it was frustrating and a bit embarassing when it was doing all that flashing and not taking a shot. This would happen even in seemingly not too bad indoor light.
Newer cameras and more expensive cameras are a lot better at this. Look at the specs on the lowest EV (exposure value) at which a body can autofocus. Older ones needed an EV of 1 to work. Newer rebels can af at ev0. A 5d3 can af at -2 ev. 6d and a 5d4 can af at -3ev. This point was a major consideration for me in upgrading and I can say the difference is like night and day. The problem basically no longer exists.
Other than upgrading, try to give the camera some help. Focus on something with contrast like a pattern or sharp edges and not something low contrast like a flat wall. Try to improve lighting in the area by turning on lights or move your subject to a brighter place.
Thank you very much... everyone has told me i have a fantastic camera. but its from 2007, wasnt originally what i even wanted, and I have been frustered for 9 years.
Your camera is rather old and less capable than the newer models but there is nothing wrong with using it for night photography. As far as I can recall, your camera does not have live view...I find that live view really makes night photography easy in many cases because in M mode, you will see exactly what you will get after you change various modes (aperture, speed and ISO). Without it , you will need to depend on the Standard Exposure Index in the viewfinder for correct exposure.
For most night shots, you will need a sturdy tripod (one that will not vibrate in the wind) and as others have stated, you will need to get off the auto modes...switch to Manual everything, include focusing. Again, live view plays an important role here also because you can magnify your image 10 times and you can really use it to precise focus (sounds like I'm encouraging you to get a newer model, doesn't it?). In low light conditions, any camera will have big troubles trying to focus and it will either tell you it can't focus (by not letting you take the shot) or worse lie to you that it can focus and give you a slightly blurred image.
So you need a tripod. You also need to be able to work in the dark (using your own light or camera lights) to operate your camera. You will need to be able to manually focus. And lastly, you need to be able to set aperture, speed and ISO to get a correct exposure. Sometimes you need to be able to calculate speed based on aperture and ISO if the speed exceeds 30 seconds--you need to use B (bulb) mode and the camera stops calculating for you. You need to know how long to hold the shutter button down. Lastly, you need a remote shutter release (cable version is preferred) to avoid camera shake when you press that shutter button. Also it will save your finger from pressing on that button for more than 30 seconds...the remote will do that for you.
The technique is rather simple - mount camera on tripod with the remote shutter release. Switch the lens from A to M, switch the camera to M. Set Aperture to f/11 or f/16, ISO to 100 and set the speed to whatever is needed to get the correct exposure...this typically takes multiple seconds but the result could be very striking like this image I took in Saigon, Vietnam a couple of years ago. Your camera is fully capable of taking this kind of shot. This was taken using f/11; ISO 200; 20 seconds. I used ISO 200 to avoid a 40 seconds exposure because I didn't have a watch (remember past 30 seconds you're on your own).