This article is from the Photographic Lenses FAQ, by David Jacobson with numerous contributions by others.
There are several ways.
a. Use a true macro lens.
b. Move the lens farther from the film with extension tubes or bellows.
c. Screw on "diopter lenses" or closeup "filters".
d. Use the macro setting on many zoom lenses.
A true macro lens generally gives the best quality. The f-number needs to be corrected according to the formulas above, unless metering is done through the lens. Most macro lenses go from infinity to 1:1 or 1:2 (mag = 1 or 1/2). Some that go to 1:2 come with an accessory screw-on lens that gets to 1:1.
Extension tubes and bellows move the lens farther from the film, allowing it to focus closer. All lenses are optimized for specific situations, and using extension tubes or bellows makes the lens operate out of the region for which it was designed, possibly compromising the quality a bit. You can compute the magnification from the extension using the formulas above. If the magnification exceeds one, it is best to reverse the lens with an adapter. Extending the lens also changes the effective f-number. See the formulas above. However, if you meter through the lens, the meter is affected in exactly the same way, so you don't need to do any calculation. Lenses on some modern electronic cameras require electrical connections to the body, complicating the construction of these devices.
Add-on lenses shorten the effective focal length of the lens and reduce the working distance. Single element add-on lenses are of inadequate quality for critical work. Many photographers report good results using 2-element lenses at small apertures. No correction is required to the effective f-number.
Some zoom lenses have special macro ranges. However, few zoom lenses get larger magnification than 1:4, and in many lenses the macro feature operates only at the short focal length end of the zoom. This is not for really serious work.
If you are photographing flat objects, such as postage stamps, freedom from distortion is important, as is a reasonably flat field.
Working distance is the distance from the front of the lens to the subject at a particular magnification. For nature work, a reasonably long working distance is important because working farther away is less likely to frighten insects, etc., and shadows are less likely to fall on the subject.