From what I remember of my D200 you need to pop up the on board flash and go to Menu > Custom Setting Menu (Pencil symbol on the left) > e Bracketing/Flash > e3 Built in flash > Commander Mode. Go to the first box right of where it says Built in and scroll down until -- is entered in the box. You are now able to fire a remote gun without using the onboard flash although the pre-flash will still fire.
Now take your SB-600, press ZOOM and the "-" button at the same time for about two seconds and new information will appear in the window, press the + or - buttons until the wireless symbol appears which is a like a squiggly arrow, above it should be the word OFF in capital letters, press the mode button once and this will change to On, now presss the ZOOM and "-" button again for a couple of seconds and the window display will change to show that you are now in wireless mode. Make sure your Channel (CH) and GROUP are the same as your camera, for ease of use I would set your Channel to 1 and your Group to A. You are now in wireless mode with full iTTL functionality without the need for any cables. You just need to make sure that the range is not too far and remember that the IR sensor thingy on the side of the flash gun (small, round and next to the battery chamber) is pointing towards the camera.
Alternatively, you could RTFM.