If you really want to buy a mid or high end SLR, you should already be at least semi-serious about photography. Otherwise, you won't get as much out of your purchase as you should. If you're just looking for something to take snapshots of friends, then do yourself a favor and either get a high-end compact or maybe a low-end SLR.
Now, let's assume you are at least semi-serious about photography? What type of art do you like to produce? Sweeping landscapes? Portraits? Sports shots? Birds/wildlife? Gritty street/personal shots? Architecture? Macro/closeups? All of them have different answers for what type of lenses are best for you.
In general, you can pick lenses that are mediocre for a lot of tasks, or ones that can do fewer tasks but do those really well.
For any particular photograph, there are two properties for a lens: focal length and aperture. A smaller focal length is wider-angle, and a larger focal length is narrow-angle. Small focal lengths also amplify perspective, while large focal length make the image seem flatter. 50mm has the same level of perspective as the human eye. A zoom lens can select from a range of focal lengths.
Aperture is the amount of light the lens lets in. A small (wide) aperture like 1.8 or 2.0 lets in more light but has less of the image in focus. A high (narrow) aperture like 8 or 10 lets in very little light but has most of the image in focus. Either your camera chooses the aperture for you or you choose it from the controls on the camera. Lenses are rated by the widest/smallest aperture they allow. A minimum aperture of 4 is good, and a minimum aperture of 2.8 is better because it gives you more flexibility.
If you're really into wide-angle landscapes, you really want a lens that can do well under 30mm (full frame sensor) or 17mm (crop sensor). In the Canon world, the good lens for that is 16-35/2.8L. It also works well in confined spaces and low light, like wedding receptions. But, the perspective distortion from 16mm makes people look funny, so you have to be further away.
If you want to do people/street photography, you want 50mm-100mm. Though I've also been able to get a lot of good outdoor reception shots from across a gathering with 100-200mm. The advantage there is there's no camera nearby so people relax. Image stabilization also helps here, since you might have to react faster and move around a lot.
If you want sports or wildlife photography, you want 300mm or greater. Be aware that a high end 400mm lens is more than your entire budget.
The kit lens for a 7D is a 24-105/4L IS. I have that lens, and it's very good for portraits and street photography and good for landscapes. I actually have that lens on my 5D most of the time, with a crop body and a 70-200/4 for mild telephoto shots and some primes for lower-light shots. I don't do sports or wildlife photography, or I'd want a still longer lens for that.
For most general-purpose people shots, the 24-105 that comes with the 7D works very well. Since the 7D is full frame, that's wider (with less distortion) than the 17-88 mentioned before on a crop body.
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