http://www.nikonchat.com/d3000-nikon/

How do I change the shutter speed of a nikon d3000?
I take photos at night with a long exposure. How I can achieve with my Nikon?
You can adjust the shutter speed in shutter priority mode (S) or Manual mode (M) [with] the opening. The manual explains how to do it in both modes. See page 69 and 71. If you do not have a manual you can see the PDF version below. http://nikonusa.com/pdf/manuals/noprint/D3000_ENnoprint.pdf
Nikon D3000 Review (detailed)