NOOOOOOOOOO Don't put it to Green (Auto) Mode!!!!! Stay in Av mode!
First of all, consider getting a flash. But anyways, here is some step-by-step advice:
- The T3i has a fair-ISO setting of ISO793, so set at ISO800 noise should be acceptable.
- Consider screwing on a nifty50 if you have one and if you don't need to zoom. Anyways stay at aroud f/4 or lower (but not too low like f/1.8) with the aperture.
- With ISO 800 and a fast lens, you should be able to get 1/15th or 1/30th of a sec for shutter speed in average-low light. Maybe even 1/60th with f/2. (Consider using a 35mm lens to get less camera-shake in the pictures.)
So Flash if possible, but if not:
MODE: AV!!!
Sensitivity: ISO 800
Focal length: 35mm or 50mm
Aperture: f/2 or f/2.8
Shutter RESULTS: 1/60th or 1/30th (or sometimes 1/15th).
Plus 1 thing: you may wish to try to add the built-in-flash to your exposures, WITH 2ND CURTAIN SYNC ... AND: Flash Exposure Compensation -2/3 or -1.5 stops. A little fill/freezing flash is better than no-flash in lowlight.
The best reason, not to go "green mode": you can't edit as well as in Raw mode. Shoot in Raw, and you'll be able to turn the lights up more easily than in JPG.