I don't recommend extension tubes. That do not allow for infinity focus and are very challenging with very little benefit. Especially with the 100mm. If you want super magnifications though, they can do a lot with a 50mm lens. The canon MP65 lens will do this with no extension tubes, but it's expensive. For normal 1:1 macro, the 100mm is perfect all by itself!
I second the suggestion for a 430ex (with a canon you'll maintain the ETTL for exposure) with a lumiquest softbox or other diffuser. With the 100 mm, this works very well mounted directly on the camera due to the minimum focus distance of 6 inches. You'll want an off camera cord eventually for lighting control. Like someone posted, the 430 is more versatile so start there. Down the road if you really like it, invest in a macro flash.
A tripod isn't necessary if you use external lighting. I never use one. However, if you want ambient light and no flash, you'll need a tripod. Unless you use high ISO to get the shutter speeds up. Bugs can be very hard to get if your shutter speeds are too slow.
DOF is very thin with macro, so you'll have to close the aperture (higher number) in order to get enough DOF. This means you need more light to get your shutter speeds up, hence the need for flash or LOTS of ambient light. With that said, some folks like very thin DOF for their macro which makes the requirement for light not as critical.
If you use external light and want to hand hold, I suggest the following settings to get you started:
Manual Settings
Shutter=200
Aperture=f8-13
Focus mode=AI Servo (helps keep things focused with thin DOF)
Focus Point- Center only
ISO=100
Hope this helps