This is what I've found so far (also being a relatively newbie to DSLR's).
Things you have to have (that don't necessarily come with the camera):
1) A memory card. Even if it's only as small as a 2GB card, you have to have at least one. Depending on how much you plan on shooting at first, one card could very well be more than sufficient. You can fit anywhere between 300 - 800 pictures on most cards (depending on the size of the photo, plus if you go with JPEG or RAW shots). If you plan on doing mostly experimenting the first few months (no big trips or anything), then one card would easily meet your needs.
2) A camera bag. You have to have something safe to keep your camera in. Bags can be as simple as a 15 dollar deal that will fit your camera and one lens, or a couple of hundred of dollars for ones that do so much more.
Things which you will want to get pretty soon:
3) A good photo editing program. Most cameras come with some kind of program that you can use to get started, but honestly, there is so much that you can do now with your pictures that a good program is almost a necessity. Good programs can run you from about 50 dollars to a couple of hundred. I use Corel Paint Shop Pro Photo Ultimate X2 which I got on
Amazon for only 49.99. I know it's not quite as powerful as Photoshop, but for the price it really does offer a ton of great features. (including RAW processing and HDR photo merge).
4) A Tripod. Even if it's only a beginner's model. There are a lot of features that your camera offers that you pretty much need a tripod to take advantage of (night time / low light shots, Exposure bracketing, self-portraits, etc...). A tripod gives you many extra photo ops that you would otherwise miss.
5) A UV / protection filter. A small investment to help protect your lens.
6) Cleaning supplies. Again a small investment to help keep your camera in the best possible condition.
Other things that you will probably also want to eventually get (but you probably won't need right away)
- an extra battery. Again, depending on how much you use your camera, an extra battery might, or might not be necessary.
- a remote control (for the shutter release)
- different lenses (telephoto, low light / fast lens, etc...)
- more memory cards
- bigger (better) flash
- A SD card reader (if your computer doesn't already have one). Sure you can connect your camera to your comp via the included cable, but it sure is easier just to pop the card out of the camera and right into the computer.