What macro lens can I get for my Nikon D3100?

My mom got me the Nikon D3100 for christmas this year. This will have been my first upgraded camera since my little compact digital camera, so I'm a beginner. I'm in high school and am very interested in photography. I know that the styles of photography I like include Macro, night/low light, portrait and spontaneous photography, however I'm most interested in macro. (mostly of inanimate objects, not bugs and whatnot)
I heard that under 100mm is better for things that are inanimate and that over 100mm is great for detailed living things, especially since if the "subject" gets scared at the fact that I'm crouching to get the shot it could run away. I'm more interested in a shorter lens but if that's relatively not possible then let me know!
So basically I'm asking what additional lenses I can get for these types of things that I'm most interested in? (aside from the lenses it came with) Of course I would gravitate towards the lower prices, but getting at least an idea is very helpful. And if you have experience with these things, opinions with that is welcome too!

Believe it or not, the 18-55 that came with your camera is great for macro. The 50G 1.8 is not great for macro photography, but it will (obviously) be good for low-light situations.

What's your budget? Nikon 40mm f/2.8, 60mm f/2.8 and 105mm f/2.8 would be great lenses.

Here's a Nikon Micro Lens Guide - http://www.the-dslr-photographer.com/2011/07/buying-a-micro-lens-nikon/