Do you have a particular budget?
A good budget lens for portraits would be the
Nikon 50mm f/1.8. It would serve you well in low light and for close portraits.
I'm not as familar with Nikon lenses since I use Canon. Try looking for a lens that can be used in low light or perhaps has image stabilization since you will be taking a lot of handheld shots. I've used a Canon 17-55 f/2.8 IS for weddings with good results. A lens that can cover some distance like a 70-200mm would be helpful, too.
I'm sure you'll get more suggestions.