Erm... 10-15secs are too long and at wider aperture you are allowing way too much light into yr camera's CCD sensor which definitely will produce undesireable results i.e. over-exposures, blown-up highlights on street lights and building lights, and un-natural sky/clouds colors (very brownish).
Personally my usual practice will be at f/5.6 - f/8 will be just nice which also subject to the surrounding light environment. If the surrounding lights are too bright, use a smaller aperture with the smallest at f/11. Always avoid direct shoot at flood lights, spot lights & tungsten lights. Choose a good angular shot and compose it from there. Well you can choose to shoot at different modes i.e. Shutter-Priority (S), Aperture-Priority (A) or Manual (M). Never use Programmed-Auto (P) mode. Personally I use M mode which I can have more controls over the shutter speed and aperture.
A sturdy tripod is a must and a cable release or wireless remote control to help reduce camera shakes. Use the lowest ISO, for yr case, Nikon's lowest ISO is 200 (preferably), for fine, less noise results. The highest you can adjusted to is ISO400.
Since you're shooting digitally, it's good to slightly under-exposed rather than over-exposed as you can adjust it in Photoshop. If the sky/clouds still look brownish, use the Burning tool to burn in the sky to make it darker. Use Level (adjustment layer) to adjust the Shadows, Mid-Tones & Highlights of yr subjects followed by Color Balance adjustment layer. If you do not know how to use or control the Color Curves, then don't use it. You might accidentally alter the "true" colors.
The above served as yr reference and it's still up to you to go out and do all the shootings and post-processing yrself to get the best results you can. From there you'll learn more and faster.
Hope my 2-cents worth of info helps. Most importantly have fun!