"However, I would like to buy a lens which will have a zoom facility "
The focal length of a lens, in mm, is how much it magnifies. To be maximally confusing, lenses for DSLRs tend to be sized in two ways: their true focal length, as you would measure it in a lab, and the equivalent focal length to get the same picture on a film SLR ("35mm equivalent" or whatever).
On a film SLR, the classic standard lens is 50mm, the classic wide-angle is 28mm, the classic telephoto for portraits is 135mm and the classic for sports and so on is 210mm. The classic zooms are 35-105 to replace a standard lens and 70-210 as a telephoto zoom.
On an EOS 500D equivalent focal length is 1.6x the real focal length, so the 18-55mm zoom lens your camera came with is equivalent to a 28-90mm zoom on a film camera - wide angle through to medium telephoto.
The other number is the f stop, and this is where the money starts to come in. The f stop indicates how much light the lens gathers, and small numbers are better. The smaller the number of the f stop, the brighter the view finder will be and the faster the shutter speed will be. Zoom lenses are "slow" in that they will be something like f/4.5 at the shortest end and f/5.6 at the longer, for standard zooms like the one you presumably have, and cheap telephoto zooms can be horror shows like f/8. There are a lot of things that make the f stop up, but fast lenses needs big pieces of glass at the front to gather all the light. Those huge lenses sports photographers have that cost six grand are only about 450mm or 600mm equivalent, and you can buy a 600mm lens for a few hundred quid; the difference is that their's are f/2.8 or something, whe the cheap ones are f/8. That's the difference between a 1/30th of a second shutter speed (useless for sport, and you can't hand-hold it with a long lens anyway) and 1/250th of a second (much more like it).
Sadly, sensibly priced, sensibly fast prime lenses are a thing of the past, and almost all consumer lenses are zoom lenses. Which means you have the weight and size of the "long" end even when you're using the short end, and they are slower than a slow thing at the long end. You want something like a 70-300, as fast (as small an f stop number) as you can afford.