hi everyone,I know this question bores you
but I am a starter and doesnt understand f:2,8 or 33 mm or macro..
you should have bought the kit lens, but since you didn't, it's not too late to do so.
http://www.bhphotovideo.com is a highly reputible dealer, they have the kit lens (EF-S 18-55mm f/3.5-5.6 II) for about $100-$150 US. more expensive than if you bought the lens with the camera, but cheaper than any other lens you will find out there.
to answer your questions:
f/2.8 is the aperture. a larger aperture means more light enters the lens. more light entering the lens = faster shutter speeds. also, larger aperture means shallower DOF (blurry background and foreground). a smaller f/stop number means a larger aperture. on a lens, the aperture is the lens' MAXIMUM aperture. in other words, an f/2.8 lens is "faster" than an f/4.0 lens. of course all lenses can be "stopped down" to smaller apertures.
33mm is the focal length. prime lenses have a fixed focal length. zoom lenses can change their focal length. think of your P&S camera's zoom mechanism. it goes from wide angle to telephoto. on your XTi, 17-18mm is considered wide angle, 30mm is considered "normal" and 50mm and up is telephoto. if you want a standard zoom lens with a wide angle, try to find one that starts off in the 17-8mm range, like the EF-S 17-85mm ... or the EF-S 18-55mm ..., or the sigma 17-70mm. lenses with longer focal lengths on the long end will obviously "zoom in" more. for instance, 85mm is more "zoomed in" than 55mm.
macro defines the lens' ability to close focus. a true macro lens can close focus on an object to a 1:1 magnification -- that means that the image will be lifesize on the sensor. true dedicated macro lenses are typically prime lenses (do not zoom in or out) and cost a little extra. macro lenses do focus to infinity -- meaning they can be used as normal lenses, too. some lenses claim to be "macro" even though they only focus down to 1:2 magnification. although that is technically "macro" only true macro lenses can focus down to 1:1
