Three principals of photography, ISO, exposure, and aperture.
Exposure refers to how long the shutter's open, longer exposure, the brighter the image, but if your exposure is too slow, you could end up with some motion blur. 1/60th of a second is about as slow as you can comfortably get for shooting things that will be moving, unless you're shooting something fast like sports.
Aperture refers to how wide the opening is at the 'iris' of the lens. A lower aperture will let more light in, but you'll have a smaller depth of field. Depth of field refers to how wide the margin of focus is. A high aperture will let less light in and you'll get a wider depth of field, but you could end up with a much longer exposure.
The last thing is ISO, ISO refers to how light sensitive the film or digital sensor is. The more light sensitive it is, the brighter the image. However, more light sensitivity is accomplished by loss of fidelity, since a high ISO, something like 6400 will have fewer light sensitive elements to allow a shorter exposure. This can be easily defeated with a bit of post production.
You camera will have a setting called 'Av', which is 'Aperture Priority Mode', it adjusts the exposure based on the aperture. Set the program dial to Av, set the aperture as low as it will go (there will be a dial in front of the shutter release that will raise or lower the aperture while it's in Av), set the ISO to 3200, (I don't know how to change this on a T2i, you'll have to figure that out) dial in the focus manually if it's too dark for the autofocus to get a bead on what you're trying to shoot, and release the shutter.