A Nikon D90 is a great camera for the price. You made a good choice. Now that you have a Nikon, you are a "Nikon person." Meaning that you will spend a lot of money over the years on that system - and you will defend your choice to Canon users for years to come. Nothing wrong with that. I am a Nikon guy myself.
Sigma makes a great 105mm macro lens for 1/3 the cost of a Nikkor. I suggest getting it. Even cheaper is the Tamron 90mm macro 1:1, which is also a great choice.
A lenses f-stop determines the aperture and depth of field. The lower the number of f-stop, the more light enters the sensor. This is a good thing. However, most reef aquarists shoot at f8 to f11 to get a blurred background and sharp foreground. It just depends on your application. Portrait photographers might want a f2.8 for portraits etc.
