Lenses for portraits, you ask? Which ones to use?
I usually suggest:
- Prime 50mm on crop camera, or 85mm on a full frame camera, for head and shoulder shots. A prime lens is good for this while being fast, sharp, and light.
- Prime 35mm on a crop camera, or 50mm on a full frame camera, for half body shots
- 16-35 zoom on a crop camera, or 24-70 on a full-frame camera, for general purpose portraits from headshots two groups of two or more.
- 70-200 for studio headshots and fashion.
The faster, the better. f/2.8 lenses are better than f/3.5-5.6 lenses. f/1.4 lenses are even better.
The affordable, great 50mm f/1.8 is a lens you should own (if you have a Nikon D40/60/3000/5000 you need to focus it manually). If you don;t have one yet, go get it.
This quick guide, which works for most photographers, should start you off well enough.