Hi,
I'd second the suggestion for extension tubes. The Kenko set or Adorama set are the two best deals for these. You can use tubes with many different lenses, for all sorts of close-up and macro work. Experiment with them.
If you do go with tubes, one thing is important if you ever plan to use them with any EF-S lens... Check that the set you get is EF-S compatible. Some aren't.
There are many macro lens possibilities, and they will work quite well with extension tubes, too, should you ever want more than 1:1 magnification.
For use on any of the crop sensor cameras, I'd suggest lenses in the focal lengths from 60mm to 105mm for most macro shooting.
Shorter macro lenses are useful for copy work - stamps, coins, etc. say - but put you too close to live critters (might scare em, or get bit or stung), and you can cast a shadow on them too easily.
60mm might be a bit short for my tastes, but okay for you. Canon's is great, but it's an EF-S lens, only usable on the digital Rebels and 20/30/40D.
70mm is a bit more like I'd want to use, personally. SIgma's is excellent from all I've heard, but I haven't used it. It is a full frame or crop sensor lens.
90mm is a good length on crop sensor cameras, Tamrons is great... At least older versions I still have are.
100mm is what I use now on my crop sensor cameras. It gives you some distance from your subject. I use Canon's, but everything I hear about Tokina's is very positive as well. Even the inexpensive Phoenix is said to be pretty good.
105mm is very close to the same, and available from Sigma. Also a lens I've heard a lot of good things about.
150mm (Sigma), 180mm (Sigma, Canon, older Tamron) are simply too long to be comfortably used on crop sensor cameras much. These almost require a tripod. I have the Canon and love it, but use it a lot less on my crop sensor cameras. The reason is simple... depth of field is razor thin with these lenses wide open. So, you have to stop them down quite a bit. As a result, you'll need slower shutter speeds. They are already pretty long to try to hand hold, and slower speeds make that even more difficult. So, while they are great lenses and all, I'd still not recommend them for use by someone starting out shooting macro with crop sensor cameras.
Now, yes, the Canon 100/2.8 is an internally focused lens. This means it doesn't change physical length when you focus it. Some of the other lenses mentioned are not internally focused, and they "grow longer" when focusing... In fact significantly longer to achieve full 1:1 magnification.
However, the effect of internal focusing is that the true focal length of the Canon changes as you focus. By the time you reach the closest focus, highest magnification of 1:1, the Canon 100 is effectively a 70mm lens. This fact puts you closer to the subject with the lens. Lenses that aren't internal focus don't change focal length, so you stay farther from your subject.This is not such a big deal with a 100mm IF lens, would be more of a concern to me with a 60mm IF lens (like the Canon EF-S 60/2.8).
I suggest you also look for books about macro shooting. Some by John Shaw and Tim Fitzharris come to mind, in particular. There is lots more to learn, such as special focusing techniques, working with flash, etc.
Have fun!