well you have a good Camera to start with.
Lenses are very important for a camera. each time you look some lenses, pay attention at the F stop.stop the lower the number (bigger F stop) the better the lens is. for a zoom lens starting at f/2.8 and up. a prime lens like 50mm f/1.4 is very good. and of course the higher is number the price is higher too.
my question for you is, do you prefer Nikon lens or a 3rd party lens?
Nikon lens might cost a bit more expensive than the 3rd party lens.
try to get a 17-50mm f/2.8 it is a walk around lens, and You won't regret. i own one for my canon 50D
tamrom 17-50mm f/2.8 is about $400
oh if you buy from a 3rd party, make sure the lens is for Nikon. because they made the same model for Canon and Nikon or some other brand.
if you need a zoom lens or macro, then you need to spend more money on it.
Note: that is no one lens can do everything.
let me know if you have any questions, i will try my best to help you out.