Hope to see you post an image soon.
If you're new to photography, using a macro lens is going to complicate things a tad for you. I prefer to use one myself when shooting critters in tanks, but due to their nature they have some characteristics that can make them challenging to master discouraging many new aspiring photographers. Depth of field ((DoF) the amount of subject matter in apparent sharp focus, or lack thereof), can be frustrating to deal with when first starting out with a macro. DoF should be your friend, it can be creatively used to add drama and effect to your images.
Yes, you want to shoot in RAW mode as a JPG is highly 'color compressed', you'll never be able to properly color correct a JPG image.
It helps if you can properly set the white balance setting to the lighting your shooting under, check your 20D's documentation. You don't have to be on the money, but it sure helps if you're in the right neighborhood.
Adobe's Camera RAW does a nice job of converting most manufacturers RAW file formats to a more usable TIFF, PSD or JPG file.
If I'm shooting corals I turn off all filtration and power heads to make the water column as still and non-refractive as possible.
I shoot in an all manual mode (setting the f/stop and Shutter Speed) using a low ISO 100 if possible i shoot a f/22 @ 4" - 20", obviously you will need a good sturdy tripod and a remote shutter release. If you don't have a remote release cable you can simulate it by using your cameras self-timer feature.
Starting at about 4 second exposures you can use your cameras Histogram feature (I'll let you look that one up in your users manual) to get an exposure that 'fills' the scale. If your image is too dark add more time onto the exposure and visa versa if it's too light.
Just my $0.50 worth, hope I didn't confuse you any more ...
Here's an image of my Frogspawn Coral, I shot it using my Nikon D200 with a Nikkor 105mm Micro lens (similar to yours) I images at ISO100 f/22 @2.5 seconds, converted with Adobe Camera RAW and I did some (very little) editing in Adobe Photoshop CS5
Good luck, and most importantly have fun!
Looking forward to seeing some of your images.