You need to know ahead of time 1) what kind of photography you're going to do and 2) how you're going to use your photos. You need the feature set that matches your photographic interests, and you need image quality sufficient for your printing needs. Let me say this on the second point. Unless you need to crop your images a lot (like in many bird photos, etc), pretty much any current, entry level SLR will let you print large prints, assuming you're using a decent quality lens.
- Bottom Line. If you have a digital SLR made in the last 4 years or so, it's probably just fine for most photographers. Later and greater models have more bells and whistles, and some of them are very useful for certain kinds of photography, but if you have a camera with about 10 megapixels (even 8 may be fine), you are likely going to be able to take great pictures with it. I use a 10 megapixel Canon EOS 40D, which was released in 2007. There have been two generations of cameras that have come after it (the 50D and the 60D). I don't think the latest in the line (the 60D) is superior enough to make it worth the jump to a new camera, so I'm sticking with the one I have for now.
- Sensor Size. The big difference between most consumer SLRs (less than $2000) and more expensive models is the sensor size. Most consumer grade SLRs have an APS-C size sensor (often called "crop sensor" cameras), and more expensive cameras often have larger "full frame" sensors that are the same size as 35mm film. This is why, if you're using a crop sensor camera, you should multiply the focal length of your lens by 1.5x (Nikon) or 1.6x (Canon) to get the effective focal length of your lens on that camera. Crop sensor cameras generally have more noise (or grain), so when you are shooting at higher ISOs, your photographs will have more grain than the same photograph taken with a full frame sensors. Full-frame sensors have a big advantage over crop sensor cameras for those who need to shoot in low light (high ISO) or need to crop photographs significantly.
- Megapixels. Most beginning photographers look to the number of megapixels as the most deciding factor in buying a camera. This is unfortunate. An image of 10 megapixels is more than adequate for most uses, including relatively large prints. I have printed 10 megapixel photographs on my wall as large as 20" x 30." You really do not need more unless you have to crop your photos a lot. In some of my bird photography, I have to crop my 10 megapixel image down to 2 megapixels to get the bird large enough in the frame. This, of course, severely limits how large I could print the photograph. If I had a larger megapixel camera, I'd be able to enlarge my printing of these photos as well. Now I need to qualify what I just said a little. Increasing the number of pixels on a sensor requires you to decrease the size of each pixel, and that in turn creates more noise. Eventually, you reach diminishing returns when adding more resolution to the camera. You record larger images because you have more megapixels, but the pixels are noiser, so they don't improve image quality as much. So, my 40D camera has 10 megapixels, and the latest 60D has 18 megapixels, which is great. But 80% more pixels doesn't translate in to 80% better image quality.
- Frame Rate. Certain kinds of photography benefit from being able to take pictures quickly. You want faster frame rates (6 fps or more) if you are going to be shooting a lot of birds or sporting events..
- Build Quality. If your photographic interests require you to have a camera that can take a beating, you need a higher quality camera. One of the greatest advantages of more expensive cameras is that there's not a lot of plastic in them. At the same time, if you don't need to lug around a heavy camera with you, why spend more money on the weight if you don't need it?
- Autofocus System. More expensive cameras typically have more focusing points, and the are often "better" focusing points. The more the merrier, but for many types of photography (such as macro), it's still better to manual focus, and for others (such as birds), I always choose the center (or center cluster) of focusing points.
- Movies. It's pretty much standard now for SLR cameras to take movies. Because of the large sensors on SLRs (even on crop sensor cameras), they can take excellent movies. If you plan on shooting movies with your SLR, check to see the resolution it records (720p or 1080p) and how it records sound.
- Lenses. In my opinion, it's a better use of money to buy a cheaper current camera that meets your needs and invest more in quality lenses. Photographs taken with a cheap lens on a great camera will not look as good as those with a great lens on an entry level camera.
- Brand. It really doesn't matter that much which brand you choose. Most photographers are loyal to one brand or another, and for good reason--they've invested a lot money in that system. It's very expensive to change once you start collecting lenses in a particular brand. I would check out Canon and Nikon first, simply because they each have so many lenses and accessories, and they are far more common, so you'll find more people that will know about your camera to help you out as you're learning. Aside from that, you can decide which specifications you need, and find the most appropriate Nikon and Canon cameras. Hold them in your hand and see which brand works for you. There's a different "logic" to the controls between the two camera brands. They both work, but some prefer one to the other.