I think US$500 is quite a tight budget to enter the DSLR field. However, you can't go wrong with Canon or Nikon DSLRs today. All of them produce stunning photos and provide all features that you need.

When making a photo, people online and also people looking at prints, cannot tell if the photo is made with a Canon EOS 1000 for US$500 or made with a Hasselblad for US$50000. They all look alike.

I personally would try to find a good offer for an EOS 1000 together with an 18-200mm zoom. However, if most of your friends shoot Nikon, then better opt for a Nikon so you can borrow some equippement.

Here's why I think the budget is tight: It's not the camera that makes the photo, it's the light. You will need some off camera light to make decent photos. And the sooner you learn to light off camera, the better. If your budget is extremely tight, you might just get yourself a reflector. However, the real magic starts to happen once you use Speedlights off camera. At least one speedlight, remote from your camera, connected via a long ETTL cable (in case of Canon). The long cables can be bought on eBay, but the Speedlight will cost at least some serious money.

In any case, start at the bottom end of the DSLRs, they are just as good as the big ones.

BTW: When I go to a photo shoot, then my fallback body is not even a DSLR. It's a simple Canon G10 point and shoot. It's nearly as good as the big camera's. But don't go for that as your first body.