I got a secondhand Canon T3i today but I can't seem to find what to do right with it. The photo I'm about to take looks great but when I view it, it looks 10x darker. Need help please.
Digital SLR camera viewfinders are optical: that is, they let you directly view the light that's coming in from the lens. So you're looking at the scene as it is.
But that doesn't mean the picture you take is going to match what you see in the viewfinder. Your exposure settings can result in something quite different.
If you want a WYSIWYG view of the image with the exposure settings you have in the camera, then use the LCD on the back of the camera with exposure simulation set, and that will be closer to what your final image will be.
If you prefer to use the viewfinder, then check your meter. It's the scale in the bottom of the viewfinder, and how it behaves changes with what shooting mode you have set. In the automated modes, the scale is an exposure compensation scale, and by default, if you don't change anything, the "needle" will be set to 0 automatically by the camera adjusting the exposure settings for you. If you are in M (full manual) mode, then the "scale" just shows you the current meter reading, and you have to make adjustments to the exposure settings yourself to move the needle.
Where the needle sits tells you if the picture will be darker or brighter. Negative EV values (on the left of the 0) mean darker, positive ones (to the right of 0) mean lighter. I suspect you have unknowningly set the exposure compensation to underexpose on the camera, by turning the wheel next to the shutter button.
See also: T3i user manual.