There's a great site I recommend to folks trying to figure this out:
(disclaimer: I'm not affiliated with it in any way).
it has all of the background info and general instructions. The Rebel XT is a camera that was introduced before Canon started using the automated sensor cleaning, so it's going ot require maintenance, but it's not a dust pig like some (my 30d attracts dust like crazy). Getting in the habit of blowing your sensor cavity before every shoot is a good idea.
Get a good bulb blower, like the rocket. Use it. I blow and check my sensor before every photo session on the 30d. By doing so, the dust doesn't stay on the sensor long and isn't as likely to pick up moisture, and it's the moisture that will "weld" it to the sensor and require a wet cleaning to remove.
Wet cleaning is scary, but if you're careful and follow the instructions you can do it. After you've done it a couple of times, you'll feel more comfortable, but it's not something to get sloppy or complacent with. I use the sensor tools described on that site for wet cleaning. I also own a illuminated loupe from visible dust (briteview, a 7x loupe) to inspect the sensor, and I find it quite useful because dust is small and the chamber is hard to light. There are a number of ways you can do that, but trying to examine the sensor with the naked eye will be tough to successfully clean.
I would strongly (STRONGLY) suggest you have the camera professionally cleaned at this point; a good camera shop in your area (especially if they have a rental area or a repair shop) can do it. If you ask nicely, many times they'll let you watch and explain how to do it; if they do, buy your cleaning materials from them for their time... Professional cleaning isn't expensive and is a good suggestion a couple of times a year in any case; I carry wet cleaning materials for an emergency cleaning on the road when I don't have many options, and will let the pros clean it for routine maintenance. If you get into the habit of good blowing hygiene, you may find you never NEED to wet clean the sensor.
Get into the some habits when changing lenses; always hold the camera with the chamber facing down, use gravity to your benefit. Have everything ready and leave the camera open as short a time as possible. Avoid doing it outdoors in dusty and windy locations and shield the camera from the wind with your body. The more you treat the sensor area as a "clean room" environment, the better you'll be. tough with one body and multiple lenses in the field, but if you think how to minimize "open time" and practice changing lenses, you can limit the problems.
And get really paranoid about blowing out the chamber. I've found it works better if you blow it out just before starting photography; I sohuld probably blow it out before AND after, but since I'm not having problems with sticky dust, it seems before is good enough..
Never touch the sensor with anything (except a sensor cleaning tool with a fiber-free cloth on it when you're wet cleaning); if you touch it you'll leave residue (skin oils, even) that'll impact images until it gets wet cleaned. So be careful poking around in there...
Make sure your battery had a good charge; if the battery fails while cleaning, the mirror will pop down and you'll likely damage the camera. Don't push it, swap batteries if you aren't sure.
(you could, by the way, be seeing a spot of dirt that's gotten onto the focussing screen and not the sensor. If it shows up in the viewfinder but not on your images, taht's what's going on; that you can ignore, but if it annoys you, you can get it cleaned; that's something again you want to let thepros do until you're comfortable doing this stuff yourself...)