Generally speaking, for low light action shots without a flash:
1) set your ISO as high as you are comfortable. (Higher values will give you brighter pictures and more noise.) (For the T3i, I would start at ISO800.)
2) if indoors/under artificial lights, set your White Balance appropriately. (e.g. "tungsten")
3) set your image file format to RAW or RAW + JPEG (so you can adjust exposure and white balance in post).
4) set your camera to Shutter Priority (or set your aperture as wide as it will go) and set your shutter to a speed fast enough to freeze the action. (For Tae Kwon Do, I would start at 1/250.)
If your pics are under exposed (which is common) you have two options:
a) slow down your shutter speed, but be aware that slower shutter speeds will have more motion blur.
b) increase your ISO, but be aware that this will add noise.
Actually, there's a third option: buy a faster/brighter (lower "f" number) lens. I would generally recommend something f2.8 or faster. The catch is that, if you can't get close to the action, fast telephoto lenses can be very expensive. If you know you'll usually be around the same distance from the action/subject, you can go with a prime lens which is generally cheaper than a zoom. If you do need a fast telephoto zoom, since Tae Kwon Do takes place in a relatively small area (as compared to soccer, for example) (read: you don't need particularly fast autofocus), you might take a look at the Tamron 70-200mm f2.8
HTH - Mark