I exchanged de canon 40 D.?

Yeah, I went to circuit city, and exchange it for the XTI, no problems at all, is a relief really because $2000.00 is to much for me. I got the XTI, still a good camera right?. I `m not an expert, but I read photography magazines and I know everything about aperture, f stops, iso, etc, etc, and I`m taking a photography class at a community college. Now I have money left for another lens, what do you recomend? the XTI came with a 18 to 55 lens, I guess a telephoto is next, right?

Don't rush, take your time choosing your new lens, it will last longer than the camera body. Buy the best you can afford.
As for which one? Well that depends on the type of photography you plan to do. Macro work, nature, portraits?
If a zoom is what you're after and you can manage it financially the 70-200mm is usm from canon is a good piece of glass.

You may want to add the Canon 55-200mm, zoom. That will give you a lot of great flexibility.

Don't forget the UV filters for both lenses to protect the front element of your nice new lenses. While lens shades can provide some protection, they break a lot easier than a filter and are more expensive

Good for you! I'm glad you were able to exchange. I love my XTI!

I have two other lenses that I carry with my XTI, a telephoto (Canon EF 70-300mm) and a macro (Canon EF 100mm).

Nice to hear you were able to work things out.

IMO you should concentrate on learning all about your new camera and postpone adding any lenses until you have exhausted the capabilities of the lens you have. Plus you'll discover what area of photography you like best. Macro? Portraits? Sports? Landscapes? Architecture? Once you've done that choosing lenses is easier.

More great photographs have been taken with a plain old 50mm lens (commonly known as a "normal" lens) than with all the other focal lengths combined.

Invest in a good tripod (if you don't have one) and the Skylight or UV/Haze filter to protect your lens and a good (Tiffen, B+W, Hoya) circular polarizer.