Since this will probably get moved anyway I might as well try to answer your question. If you are thinking of buying a used SLR (such as ones on Craigslist), be sure to ask for a copy of a full-size recent photograph taken by the camera.
Then you can upload it here:
http://regex.info/exif.cgi
and you can check for the number of 'shutter actuations'. dSLRs have a camera life, the mirror inside that clicks when you take a photo moves everytime you snap a photo. Eventually it will break.
My camera currently has 15,000 shutter actuations, and I predict it will probably die around 40,000 -50,000. Some people's camera have lasted way long than that. Pro-grade level cameras can last from 150,000 - 200,000+. So if the used camera has a couple thousand shutter actuations you're in pretty good shape.
You also want to check for any physical damage. If there is so much a chip or a scratch I wouldn't buy it, personally. The camera may have been dropped, which is the worst thing that can happen to a camera.
Another thing to check is try an get a feel if the previous owner had switched lenses a lot. This introduces dust inside the camera, which will land on the light collecting sensor, and it may show in the photos. Check for dust in the view finder too.
If the camera is fairly new try and get a copy of the receipt for warranty purposes. Also, as a beginner, just stick with an entry level Canon or Nikon. You have a plethora of used lenses and accesories available to you, all used, etc. Entry level because the biggest thing to overcome is knowing how to 'compose', or to place the camera in the right way and build place the subjects in the correct way.
Lastly, invest in books and read rather than try to get fancy equipment. You don't need fancy equipment to take good photos, just like how Tiger Woods could used old wooden clubs and he's still kick your ass in golf, even if you used the finest titanium clubs available. So, don't waste your money. It's 95% photographer and the 5% is up to the camera.
Fancy equipment just allows people to switch settings a little easier, but it's no replacement for proper compositional skills.