Victor Wei , Dec 04, 2010; 12:22 a.m.
You can certainly start with a very worthwhile and yet affordable investment on a Nikon 50mm f1.8 lens (at about $110 US) - but this lens won't autofocus with your camera. Alternatively, you can also consider the 35mm f1.8 DX which will autofocus with your D5000 but this lens may be too wide for great looking portraits. The 18-200 lens is very versatile and can make a decent portrait lens but it will not give you the sharpness of the other two. So it really depends on how serious you are in taking portraits as there are other great but rather costly lenses like the 85mm primarily used for portraits.