Aurora,
Generally speaking, budget will determine everything
. I'm not sure what lenses you currently have?
However I really like the Tamron 17-50: it is reasonably cheap (I paid about $750, and the equivalent Nikon is about $1500), it is a fixed 2.8 aperture, very sharp, and has Vibration compensation (Tamron speak for VR, Vibration Reduction).
If you are on a budget, I would go for a Nikon 35mm 1.8 (for a couple of hundred dollars). You will keep it in your kit forever, it has a very wide maximum aperture, and has long been the sort of lens that is recommended to beginners.
As far as general advice goes - get into the learning plans in the New to Photography section here, and get the camera off P mode!