here are a few threads you may want to check out.
http://taperssection.com/index.php?topic=33303.0http://taperssection.com/index.php?topic=34378.0http://taperssection.com/index.php?topic=31570.0fyi, i don't know your level of understanding in the camera world, but it sounds like you are somewhat new to slr photography. my apologies if this is too elementary for you, but i figure this primer would help. first, in those threads, prime lenses are lenses that are fixed focal length-they do not zoom. zoom lenses obviously zoom.
now, each picture is an exposure, which refers to the recording of light by a medium, in your case, the digital sensor. there are 3 factors that come into play in determining the exposure: aperture, shutter speed and ISO.
Aperture - the aperture is the opening inside the lens that allows light to pass through to the shutter. this is referenced by an f-number. for example, a lens rated at f4, has a maximum aperture of f4. the numbers are the denominator in an equation so a smaller f-stop actually allows MORE light to pass through. to make it more confusing, each major f-stop actually allows 1/2 the amount of light to pass through as the numbers get higher. the major f-stops are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. so an f-stop of 2 allows in twice as much light as f2.8. your kit lens is a variable aperture lens so as you zoom it, the maximum aperture shrinks from f3.5 to f5.6. at f5.6, it is virtually impossible to get a good shot in concert conditions.
shutter speed - refers to the amount of time the shutter stays open. the longer it stays open, the more light reaches the sensor, but also increases the chances for blur (it's hard to handhold and keep it steady at slow shutter speeds). the rule of thumb is that you should be able to handhold up to 1/focal length of your lens. so if you have a 50mm lens, you should be able to handhold at 1/60, 1/90, 1/125, 1/250 of a second,etc. once again, as the shutter speed gets halved, twice the amount of light is let in. for example, going from a shutter speed of 1/60 of a second to 1/125 of a second will reduce the amount of light reaching the sensor by 1/2 because the shutter is open for half as long.
ISO - measures the sensitivity of your sensor to light. your rebel has 5 ISO settings: 100, 200, 400, 800, and 1600. each step up in ISO makes the sensor twice as sensitive to light. so if you were to jump from ISO100 to ISO200, you would need to either make the shutter speed 1 stop faster (eg 1/60 to 1/125) or make the aperture one stop smaller (f2.8 to f4) in order for the exposure to be qualitatively the same. the disadvantage of higher ISO is that you will see some "noise" in the image - basically like littel bits of television snow.
so there are all tradeoffs to the settings. I use 2 primes and 1 zoom for concert shooting: a 50mm/f1.4, a 135mm/f2 and a 24-70/f2.8. among these lenses, the 50mm has the largest max aperture of f1.4.
now comparing these to your 18-55f/3.5-5.6, these lenses have a much larger max aperture than the kit lens. to use the kit lens at 18mm, your aperture is 3.5, the 1.4 that i have allows more than 4x as much light pass through to the sensor, which is obviously very useful in concert shooting.
the 50mmf/1.8 would be a worthy lens for you to pick up for shooting concerts at around $60 (camera lenses get ridiculously expensive, the 50/1.8 is a great deal). experiment with shooting in manual mode and learning how each setting affects the resulting image.
you may also pick up a book called Understanding Exposure by Bryan Peterson. It's the best book on explaining the fundamental concepts at work and he uses images that he shot to demonstrate his point. it is not related to concert shooting at all, but will help you become familiar with the different functions and features of your camera. good luck
-damon