I'm interested in getting a DSLR and lens for fashion and nature photography. So far I've been partial towards Nikons, so I would prefer getting a Nikon for my first camera.
Nothing wrong with that, however, I'd strongly recommend that you physically handle the different cameras from different manufacturers, and see which ones you prefer the feel of. Canon, Nikon, Pentax and Olympus all make good cameras, but you may have a stronger feel for one over another, even within brands.
Canon and Nikon (maybe others, I'm just not as up on them) have different "series" of cameras. As an example only, Canon's EOS "Rebel" series (if you're in the US, otherwise the XXXD series) are all built on a physically smaller chassis than their higher series (designated EOS xD or EOS xxD worldwide). That made a big difference for me, as I found the Rebels too small for my hands.
I believe that, in Nikon's parlance, the DX000s are the smaller chassis, "consumer" cameras, but I'm not certain.
Price range is 300 - 600 USD (at the very very most) for everything if it's possible.
That'll be a stretch for new gear, even at your top end. The "kit" lens you'll get at that price won't be very effective for nature/wildlife photography. $500-600 is entry-level just for a good lens for wildlife photography, where 300mm is about the shortest you want to go. APS-C size sensors help by stretching a 300mm to an apparent 480mm, or a 200mm to 320mm.
Many options for used gear, but harder to get a feel for them. But newer versions of the same series of cameras are usually evolutionary, rather than revolutionary, so if you went with Canon, say, trying on a 50D would be not that much different than trying an older 30D or 20D. I'm sure the same would be true of Nikon and others.
I'm open to any suggestions and recommendations.
1) Try on different cameras. You may find something you like that's only on one, and not another, and that may be more important to you than brand loyalty.
2) see if you can borrow one or more different DSLRs from friends for a weekend or so to get to know them.
3) check out the forums on
Photo.net and the articles there for beginners.
4) buy or borrow the latest edition of "DSLRs for Dummies". I don't normally do the "Dummies" books, but for a quick, reasonably complete (hard to do in an industry that changes monthly) overview of all things DSLR, it's hard to beat it.
Best of luck to you!

