Go to your local photography shop and handle both Canon and Nikon cameras. Figure out which feels best in your hands, which one your fingers can most easily acess all the buttons on.
While I shoot Canon, both Canon and Nikon make exceptional products. Find the one that fits you best and go with that body. THEN the fun begins. Depending on which brand you go with, now's the time to start learning. DSLR photography is A LOT different than using a Point and Shoot! There's a huge learning curve. Familiarize yourself with all of the different modes/settings and learn how each setting effects the photographic results differently.
Once you've gotten comfortable with your purchase and learned some of the basics, it's time to start shopping for a Macro/Micro lens. Canon calls them Macro, Nikon calls them Micro. Canon 100mm Macro is an exceptional lens and also doubles quite nicely as a portraiture lens. Nikon 90mm Micro or 110mm Micro are also very high quality lenses.
Macro photography requires A LOT of practice. Your FOF (field of focus) is very small and minute movements to the camera, or subject, will effect focus.