Not having a Rebel XT, I cannot answer the question as asked. However, I think you'd be better served by understanding night shooting rather than getting instructions that work for a single place and time.
The challenge with low light shooting is getting enough light on your sensor for the camera to produce a picture. There are four main things you can do to get a picture with low light.
ISO - This controls how sensitive your sensor is. The higher the ISO number, the less light it takes to make a picture. On the downside, the higher the ISO, the more "noise" or unwanted little dots you'll get in your picture. ISO for cameras typically range from 100 to 1600. Some only go to 800. Others go as high as 3200. The level of noise also varies from camera model to camera model. ISO 800 might look good on one type of camera but it might look awful on another.
Aperture - This is how wide the opening in your lens is. The wider the aperture, the more light you get on your sensor. The aperture is specified as an f-stop number. The smaller the number, the bigger the aperture. I know that's confusing, but I didn't invent the system. I think the Sigma lens you are using has maximum aperture of f3.5 to f5.6 (depending on how far you zoom).
Shutter Speed - Obviously, more light will hit your sensor the longer you have shutter open. The downside is that the longer you have the shutter open, the more likely it is that the camera will move while taking the picture. Movement while the shutter is open causes blurry pictures.
I'll try to explain it again with an analogy. Let's say that you have a tank of water over a bucket. You poke a hole in the tank so that water starts to fill the bucket. The aperture is the size of the hole. Make the hole big and more water gets into the bucket. The ISO is the depth of the bucket. If you have a high ISO, your bucket isn't very deep and it doesn't need much water to fill it. If you have a low ISO, you need more water. The shutter speed is the amount of time it takes to fill the bucket.
The fourth thing you can do is add more light. That's what your flash is for. Unforunately, there are a lot of drawbacks to flash - they bother people, they change the look of things, they cause red eye, etc. Still, sometimes you don't have any options. If you can't get the shutter speed that you need with your highest ISO and widest aperture, your only option may be to add more light.
Another thing that really helps at night is using a tripod. A tripod lets you take a picture with a much longer shutter speed. If you try to take a picture with a 1/4 second shutter speed holding your camera by hand, you are likely to get a somewhat fuzzy shot because you moved the camera just a bit while taking the shot. The tripod keeps the camera from moving at all. Image stabilizers can also help you use longer shutter speeds without blurring the picture.
The problem with tripods and image stabilizers is that they only stop the camera from moving. They don't help with the subject at all. If you take a 1/4 second shot of someone hitting a baseball, you might get a cool motion effect, but you won't get a very sharp picture of the batter.
If you can't use a flash and the shutter speed is too slow even with the highest ISO and widest aperture, the only option left to you is to increase the maximum ISO or widest aperture. That means either using a camera with a higher maximum ISO or using a lens with a wider aperture.
