The focal length of a macro lens is all about the working distance between your camera and the subject. It's not about the strength of the macro magnification. Most macro lenses today are 1:1, which means they'll focus close enough to render the subject lifesize. On your camera, that means you can photograph an object or area approx. 15mm x 22mm... full life size.
Shorter macro focal lengths (35mm, 50mm) put you pretty close.
Longer macro focal lengths (100mm and up) give you more room to work.
There are times you want to be close, such as when shooting tabletop studio stuff, so you can reach out and adjust the object you're photographing, while keeping your eye to the viewfinder.
And there are times you want to have more working distance, such as when photographing skittich small critters, anything that bites or stings... or to help prevent accidentally casting a shadow over your subject.
But, the longer the lens, the harder it is to hold steady. Depth of field is very shallow shooting macro, so you often have to use smaller apertures (f11, f16, maybe smaller though you risk diffraction causing lost of image quality). Smaller apertures mean even slower shutter speeds, making holding the camera steady even more challenging. Of course, to some degree you can offset this by increasing ISO... Or you can add light to the scene with a flash.
I usually recommend a 70mm to 105mm lens as a good, all around compromise when starting macre... but there are two good, general purpose 60mm, too, which will be more compact and might be long enough for a lot of folks, depending upon how high magnifcation they actually use and what sort of subjects they shoot. Even with these limitations, there are a lot of choices:
Canon EF-S 60mm f2.8 (this is a crop only lens)
Tamron SP 60mm f2.0 (unusually large max aperture, might be more useful doubling as a portrait lens).
Sigma 70mm f2.8
Tamron SP 90mm f2.8
Tokina 100mm f2.8
Canon 100mm f2.8 USM (technically discontinued, but still widely avail.)
Canon 100mm f2.8L IS (image stabilized, pricey)
Sigma 105mm f2.8 (recently discontinued, but might be avail.)
Sigma 105mm f2.8 OS (stabilized, a new model)
All these are 1:1 capable and have great image quality. How to choose between them? Look at all the features they offer, such as:
Choose a convenient focal length for your particular purposes.
Tripod mounting ring? It can be very handy. AFAIK, only avail. for the two Canon 100mm (a separately sold accessory, look on eBay for clones that are quite a bit cheaper than the OEM ring).
Internal Focus... makes the lens larger overall, but it doesn't change in length when focusing, so might balance better or give more consistent working distance (note... a macro lens has to move the focusing group a long, long way to go from infinity to 1:1). Note that IF lenses do "change focal length" when you focus them... The Canon 100mm, for example, is closer to a 70mm by the time it's focused all the way to 1:1 magnification.
Focus limiter.... This is a switch that restricts the focus within a certain range. It's probably more important if you want the lens to serve dual purpose, such as as a portrait or short telephoto lens some of the time, for non-macro shooting. The Canon 100mm lenses have a limiter. You'll have to check if the other lenses do.
USM (or similar) focus drive... This is faster than micro motor type focus drive. It's also quieter and can be more accurate in some cases. Often macro shooting is done using manual focus methods, so it might not be a super important factor unless using the lens for other, non-macro purposes. Canon's is USM. Some Sigma have similar HSM. And I think Tamron has recently started offering USD, which is similar.
I agree with Virto, the Canon MP-E 65mm probably would not be a good choice for someone just learning to shoot macro... It's an ultra high magnification, fully dedicated lens and starts where most macro lenses leave off, at 1:1 and goes up to 5:1 magnification, or five times life size. It's also strictly manual focus and would be largely a tripod-only lens. Part of the challenge working with this high magnification is that you are photographing things that can't be seen with the naked eye, so it's harder to find and compose your shots... But it can do really cool and amazing shots at very high magnifications!
I don't fully agree about the Canon EF 50mm macro... It's a 1:2 lens on it's own, which might be enough magnification for a lot of people. It's also very compact, compared to the other lenses. So, it might be a good choice for some people. And it can be used with a matched accessory to get to full 1:1... or could be used with universal macro extension tubes to increase its magnification, if needed.
