It's a combination of several things.
What's probably the most important is the lens you're using. Most portrait photographers will use a fast prime in the 50-100mm range (I don't know about Nikon, but Canon makes a mean 85mm 1.8 that is great for portraits). The important thing with the prime, apart from the sharpness, is the large aperture - it lets you nail focus down on the eyes and leave very little else in focus.
A lot of the smooth skin is probably post-processing or makeup beforehand.
If you really want to get serious about portraiture I'd suggest at least getting a nifty fifty and using that for a while. It's cheap and will let you play around with primes... other than that, look up some portrait processing techniques. Although be careful not to go overboard with the post-processing; keep it natural-looking. Enhance the skin, don't replace it.
Hope that helped, good luck!
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