Cheapest way to get into macro, (quality wise), 40mm. Under $300.
Nikon 40 mm f/2.8G AF-S DX Micro-Nikkor Lens 2200 B&H Photo
60mm. Under $500
Nikon AF Micro-Nikkor 60mm f/2.8D Lens 1987 B&H Photo Video
85mm Under $500
Nikon AF-S DX Micro NIKKOR 85mm f/3.5G ED VR Lens 2190 B&H Photo
105mm. Under $900
Nikon 105mm f/2.8G ED-IF AF-S VR Lens 2160 B&H Photo Video
Depending on how serious you want to get, you'll want to get a flash that can be triggered remotely.
Otherwise, just pick up the 40 or 60mm, a tripod and remote trigger, minimum.
Important:
Your depth of focus is so small with a macro, your heartbeat and breathing will throw the focus off. Get a tripod. Even with a tripod, you'll run into issues; any creature that is mobile will constantly be in and out of your depth of focus. Not being able to move the camera with it, you'll find yourself constantly moving the tripod. It's either a waiting game, or chasing it with the camera til you get lucky game, especially when it comes to fish.
Oh, and always FOCUS ON THE EYES. If it's a coral, focus on the mouth.
If this is your first camera setup, learn how to take a proper image first. Just slapping a lens on there and setting it to AUTO will do you very little. Learn and understand what everything does, unti it 'clicks'. It's not hard.