First make sure the camera is set up right for the conditions you're shooting in. Learn about ISO, shutter speeds and fps. T3i doesn't magically make good footage when it's turned on.
You can use some programs to import the footage from your SD to your computer for better compressing. If you do that then set the export settings to the exact same as you were shooting in (for slowmo you want to set the fps to 24 though). This step isn't necessary though as internet video players don't use the quality you gain from this step. (It is necessary for slow motion clips though)
Export from your editing suite using the same settings as you shot the footage in (here I'm only talking about size, frame rate and aspect ratio). Use Google to find out the best export settings for your editing program and camera (there are loads of settings that I don't even know what do, but they make a huge difference for some reason).
And of course, if you can get a hold on an on-camera light (for night shooting) or a stabilizing rig that makes a HUGE difference. Don't try to get away with digital stabilizing, it resizes your clip so you'll see some unwanted gain or strange flickering (dlsrs have a weird tilting sensor problem that can be seen fairly easily when paning the dslr)
Aaaand make sure you're filming with PAL/NTSC correctly (according to your home country, google this), PAL will look weird when handled as NTSC and vice versa
