Make a list of what you will photograph, how often, etc. Then make a list of how involved you want to get (do you want to develop your own, etc.). See if you want auto-focus, or not. With that list in hand, google some cameras and see which ones fit the features you need (there is a good site photozone.de, that has side by side feature overviews of most SLR's). You will surely have some questions as to what does what and what some of the numbers mean - I am sure you will find answers here if you just ask. At that point, you will have a list of cameras that fit your needs. All that remains is scanning eBay, or KEH, to get an idea of prices. That will narrow your choices further as you see what fits your budget and what does not. Since you said you had a preference for Nikon (presumably because you already have some gear), that should make things even more simple. I think that's about as close to step by step as I can think of getting

Peter.