Unlike some other branches of professional photography, you don't need to spend a fortune on equipment to get started. For a camera, almost any digital SLR or even a good high-end compact will do. Any of the current 10-megapixel digital SLRs, such as the Sony Alpha A100 that I use, or models such as the Canon EOS 400D, the Nikon D40X or the Olympus E-510 will be more than adequate.
I have also had shots from several compact cameras accepted by photo libraries, although these were top-shelf cameras with large sensors, high quality lenses and RAW mode capability. Models such as the Fujifilm S9600, Olympus C-7070 and Ricoh GR Digital have all produced saleable photos. The photo of the Dartmoor ponies on the previous page was shot on the Olympus C-7070 and successfully sold.
Apart from a good camera, the other major expense is a copy of Adobe Photoshop. There is a reason why Photoshop is the industry standard, and also why it is so expensive, and that is because its algorithms for image enhancement and enlargement are significantly better than any other image editing program. You'll need this because many stock libraries (for example Alamy) have a minimum size requirement, specifying a file size of at least 48MB. The only camera on the market that can produce files of this size without modification is the Â£6000 Canon EOS 1DS MkIII, and if you have one of those then you probably don't need my advice on anything. For the rest of us mere mortals, this size requirement means that images will have to be re-sized, and only two methods of doing this are of acceptable quality. One is the bicubic resample resize feature of Photoshop CS2 or CS3, and the other is a program called Genuine Fractals, an amazing image re-scaling program which works as a Photoshop plug-in, so you'll still Photoshop in either case.
Obviously you'll also need a computer and an internet connection, but since you're reading this we'll assume you've got that covered.