Congrats on the buy, John. The D90 is an awesome camera for a first DSLR, great choice. The 18-105 is a nice all-purpose lens that'll help you discover what type of photography you have the most fun doing. I'd buy a UV filter and nothing else right now (this filter is mostly to protect the front element of the lens).
Like Kofman said, this is not the time for a shopping spree. Leave that for later, when you know exactly what type of photography you'll be doing. (For example, the 50mm lens that Kofman mentioned is a solid piece of glass, but if you're only going to shoot landscapes then you really will have no use for it.)
Now that you have all you need to get going, the next step is to shoot, shoot, shoot. Then, post your favourite images here and see how people react. Learn from your successes and your mistakes. Then go out and shoot some more. Rinse and repeat :thumbup: