Nikkor micro lens shopping assistance please?

I'm 15 and i have recently picked up a Nikon D90 DSLR. I already own a 70-300mm lens for nature shooting but, i would love to get in close because i love those shots and i have so many objects i would love to shoot. I have seen many different types of lenses for macro shooting but not sure which one is right or my D90. Please help me pick the perfect one. My budget is about 0-800$. Something not hard to control but amazing picture quality. (for a D90) thank you so much.

I think Jens' answer is very good. I would only add that it is nice to be able to put a lens on your camera before buying to get some understanding of what it will do for you. I have a D90 also and I use the Micro-Nikkor 105mm for flower photography. But not everyone has the same same working style.

That depends on what you want to shoot. Basically you can get either a 60mm macro which will also be good for portraits but not good for shooting living insects as these would be scared away since you have to get really close to them, or a 100mm macro that will be good for insects but a tad long for portraits.

These are lenses worth considering:
http://www.adorama.com/...8AFGU.html
http://www.adorama.com/...8NKAF.html
http://www.adorama.com/...028NK.html
http://www.adorama.com/...PNKAF.html
http://www.adorama.com/...AFVRU.html

I'd get the Nikon 60mm for a short one, and the Tokina for a long one (due to the price)

Edit:
Be aware that for proper macro photography a tripod is a must, and ideally also a rail mount on it to adjust the distance without having to move the entire tripod. Count in some budget for that as well.