Jeff is right. We need to see some examples and know the settings.
Could be many different things.
I also suggest you look at your d90 link in the digitutor sight http://www.nikondigitutor.com/index_eng.html
Not knowing how to change settings is sort of like buying a fancy car without knowing how to drive. With digital, mistakes don't cost much and you'll be more able to get the results you should from a fine camera.