The Canon EF 35mm f/2.0 lens is an excellent lens for model photography (I use it on my original Digital Rebel and the current 30D). It’s 35mm, but due to the 1.6x field-of-view modifier, it acts as a 56mm lens on the Digital Rebel XT or XTi (or 30D, for that matter)–which is a good, “normal” perspective. It’s fast, compact, and very, very sharp. The 50mm f/1.8 lens is also excellent, but rather flimsily made.
The EF 100mm f/2.8 macro lens is superb, but I’ve found that it really, really, excels in drawing out all the flaws in your subjects and models. Besides, that 100mm becomes 160mm on the Digital Rebel, which makes it kinda awkward for anytihg other than an extreme close-up.
I wouldn’t recommend a zoom lens for hobby photography. Depending on the zoom range and price, the lens distortion is much greater than prime (fixed length) lenses.
If you don’t have one, best invest in a good, sturdy tripod, cable release so that you don’t always use the timer or risk shaking the camera with longer exposures, and think about lighting. NEVER use onboard pop-up flash for any kind of photography, unless it means the difference between photo and no photo.
There are basically 3 lighting options: You can use an external flash via the hotshoe, or use a tripod and your ambient room lighting and settle for longer exposures (say .5 second and up), or you can set up a mini-studio with a lightbox and a dedicated light setup. If you get an external flash, be sure to get one that has adjustable tilt-and-swivel flash head. The Canon 580EX flash can act as a master flash, and trigger slave flashes from different angles, but they add up fast (around $300-350 for the 580EX, I think ).
As with all things a lot of it is “it depends” on your situation and finances and acceptable limitations. If you have more questions, post!
P.S. One of my other hobbies is sci-fi/fantasy mi