The standard size is 50mm. That's good for general shots of people and scenery. A telephoto lens lets you take a "close-up" picture of something that's far away. Telephoto lenses are bigger than 50mm. There are a variety of sizes. A typical size is 100mm or 200mm A wide-angle lens lets you include a large area in your picture, even if you're fairly close to what you're photographing. A typical size is 12mm to 25mm. So, the answer to your question is: Get a 50mm lens, a telephoto lens and a wide-angle lens. Or....get a zoom lens that takes the place of all three. Go to a "real" camera store, not a discount house, and ask the clerk for advice. A discount house worker probably won't know enough to advise you unless he or she happens to be a photographer.

The 'normal' focal length of a lens is usually considered to be the length of the diagonal of the image on film. A full-frame 35mm image is 24x36mm, which produces a diagonal of 43.3mm. However, the 'standard' 35mm lens is normally a 50mm or, sometimes, a 55mm. Wide angle lenses usually start at 35mm and go down from there. Common focal lengths are 35mm, 28mm, 24mm, 20mm, and shorter. The lens price increases as the focal length decreases. There are several zooms on the market that offer focal lengths from under 20mm to about 50mm or from 24 or 28mm to 80mm or 100mm. There have been some very short focal length zoom lenses introduced in the past year or two (down to 12mm), but these are designed for digital SLR cameras with APS-C sensors. They should not be used with 35mm film cameras. A 12mm lens mounted on a DSLR with an APS-C sensor is equivalent to an 18mm lens on a 35mm SLR. Short telephoto lenses are usually in the 75mm to 105mm range and are often used in portraiture, because they flatten the image slightly. Longer telephoto lenses start at about 135mm. Conventional lenses are rarely made with a focal length greater than 500mm, because of the limited market for them and the high cost of manufacturing the optics. There are some inexpensive long lenses on the market, but not from the major 35mm camera manufacturers. Some long lenses (500mm and up) are catadioptric mirror lenses. Mirror lenses are cheaper to make and can be useful in some applications, but the lens design does not produce as good results as conventional lenses. Most 35mm SLR manufacturers package their amateur cameras with a lens in the 28 to 100mm range. They also provide lenses in the 100 to 300mm range, priced for the same market. These two lenses cover the needs of a large number of amateur photographers. I have several lenses for my 35mm SLRs: 24 to 120mm zoom, 70mm to 300mm zoom, 60mm macro, 24mm, and 16mm fisheye. (The 24mm lens can only be used on one of my bodies. It is incompatible with bodies from the same manufacturer manufactured after the early 1980s.) There are several medium-format SLR cameras on the market that use 120 and 220 roll film. The most common image formats are 4.5x6cm, 6x6cm, and 6x7cm. The 'normal' lenses for these are 75mm (4.5x6), 85mm (6x6), and 90mm (6x7). The 50mm lens on my 4.5x6 SLR is roughly equivalent to a 30mm lens on a 35mm camera - nice for landscape work.