I am now a grizzled veteran of event photography... I was pretty much the resident photographer for my large HS's yearbook last year. If you can get a dress rehersal, it's better bc you can move around and the like.
The 50mm lens (1.8) is decently long (about 80mm on film), and REALLY fast & sharp! Probably my best lens, and I got it for $60 on ebay. Get one.
Your 70-300 will work if it's fast, which it probably isn't, knowing superzooms.
Get a tripod, stick it in AV mode somewhere between 1.8 and 3.2, ISO 800 or 1600. Focus manually if you need to, but telling it to focus on the center point really helps it find its target. I sometimes get it to focus (bright, contrasty areas are best), then stay put and put the lens on manual focus so it isn't always hunting.
Direct flash sin't the prettiest, so if you can, diffuse it, or if you have a big flash, bounce it off something. BUT a big flash isn't generally something beginners have...
Even though the XT is good, a fast lens is crucial, and the 50 f/1.8 is a FANTASTIC bargain. It really is sharp, and it is fantastically tiny on that little body.
--------TO SUM UP------
-Main dial (left side) -->Av, aperture set to 1.8-3.2, depending on how much depth you want in focus, and how little light there is
-ISO --> 800 or 1600. 800 has less noise (grain), but you may need the extra light from 1600. It depends.
-Focus --> IF it can focus automatically, great. Let it. If not, try to get it to focus once, the set it to manual focus. If BOTH of those fail, you can manually focus, but basically, you need more light.
-also, get a tripod... you don't want shaky hands to ruin the shots.