Red eye on means the camera will make it so you don't see the inside of the eye (why some peoples eyes look red in some pictures... In animals it can look red or a greenish depending on the animal). So it is up to you if you want it on or off.
450d is the same thing as an XSi just in case you get people that use the other term. GREAT camera, good choice! IMO unless you are going pro it is the best set up for the price right now. The new ones have video but it is really bad and not worth the extra money.
yes, you have what they call the "kit" lens, it is the one that comes with the camera when you buy it (most of the time). 18-55mm is a great starter and all around lens until you figure things out. The "IS" means it has Image Stabilization... so if your hand is a little shaky the lens will fix that for you so your picture doesn't look blurry.

Glad you have a filter. the 58mm on that is the size of the threads on the end of your lens that it screws onto. The UV filter is great and I always keep one on my lenses. Mostly they protect the lens from getting scratched and they do help the picture some.

You might not have understood my question but you answered it right so you know more than you give yourself credit for!

What are you taking pictures of????