I've got an XTi and am using these lenses:
Canon 35mm f/2 EF
Canon 17-55mm f/2.8 IS EF-S
Canon 100mm f/2.8 EF USM MACRO
I like the 35mm since it is really nice standard lens to learn on but I find myself using the 17-55mm lens the most and the macro the least. The macro lens does get more of a work out than I expected however.
The next lens I want to buy is the Canon Zoom Telephoto EF 100-400mm f/4.5-5.6L IS since I borrowed one and got some really nice shots like this one below.
Another thing that you might find helpful when shooting in Av mode are these steps:
1. Choose your ISO value
* ISO100 when the sun is bright outside
* ISO400 for when it is slightly dark outside
* ISO1600 for dark indoors
2. Choose aperture
* Large if taking pictures with the available sun or wanting a shallow Depth of Focus (DOF)
* Smaller if wanting a large DOF
3. Check the shutter speed to ensure it is sufficient and your camera can handle it. If not, back to step 1.