Accessories for a Nikon D3000?

I'm an amateur photographer, still learning! I have a Nikon D3000 with the standard lens- 18-55mm 1:3.5-5.6. What type of lens should I invest in next? Maybe a 50mm? Or a more longer zoom lens? Any recommendations?

You choose lenses based on what you like to shoot - no point in collecting random lenses for collecting's sake.

If you want wide, dramatic landscapes, then something like the Sigma 10-20mm.
If you want wildlife/sports, get something longer - 55-200mm is a reasonalbe consumer zoom that would complement your 18-55mm
If you want to do portraits/low light work, get the 50mm f1.8.
If you want to do macro - get the Sigma 105mm or 50mm f2.8 macro (the 50mm also doubles up as a good portrait lens).

I have a Nikon D3000 too. I have a Tamron 18-270mm lens, and since i travel around a lot and don't want to deal with carrying around and using lots of different lenses, this works well for me. I do wish I had something with a wider aperture though.