What is your camera setup?
Macro focal lengths lenses are usually 50/70/100/150/180mm primes, with maximum apertures of at least f2.8 (or a bit more on the 180mm lens) with the longer focal lengths giving greater working distance from the subject. The most popular length is a 100/105mm macro.
Canon also have a 65mm macro lens (MPE 65 I think)that enables 1:1 to 5:1 macro ratios for ultra close-up macro photography.
Nikon have a macro lens that has VR (the only one on the market I believe).
So, in short, you want a macro lens that has a "true" macro ratio of 1:1 lifesize reproduction and a fast maximum aperture.