Was it in bright sunlight? Even at a high aperture (f/22 if you're using the kit lens) it can still be too bright.
Otherwise like Dirtysocks said above, it can be down to a number of things.
If you're aperture is set high (inversely a low number, f/2.8, f/3.5) it'll let a lot of light in. If you want to keep this high so you the model is in focus but the background is out of focus, you could increase you shutter speed. the faster the shutter speed (the time it is open) the less light that can get in.
Also, make sure your iso isn't on auto if you're shooting in Manual, and set it to 100, the lowest setting.
Additionally, you can underexposing it using the the AV button above the Q button on the back of your t4i. Whilst holding this, move the wheel on the top of the camera left, you'll notice the meters going from 3---2---1---0---1---2---3 exposure bar at the bottom of your screen or in your eyepiece. Moving the small rectangle left on this bar will make the images darker, which you can see happen in real time if you shoot in live view. You can see this on my photo below.
Untitled by
rosshmadden, on Flickr