Portrait lenses are normally considered to be those between 50mm and 135mm on a 35mm camera. There are a few of reasons why these focal lengths are often used. A wider lens distorts the face and separates the subject from the background, while longer lenses compress facial features, they require slower stops (and therefore more chance of camera shake) and bring the background right up on the subject.
There is a definite cultural comfort zone for personal space, and you will find that while many people may be quite happy to have their photograph taken they will not be able to relax if you're right in their face. Use a longer lens, move back and let them be comfortable.
I used a 50mm lens to get this portrait, left. 50mm is my favourite lens for portraits as it does allow me to get close and some of this intimacy sometimes shows in the end image. With my very limited Vietnamese I thanked him profusely. I met him later, with his wife in the market and he came up to me beaming and shook my hand. He was thrilled that someone would want to take his photograph. Naturally, I was delighted too.
Good use of lighting is an integral part of good portraiture. By being observant and using the light you can enhance your portraits and avoid many common mistakes.
Try and photograph early in the morning and late in the afternoon. Watch for especially harsh midday sun that can make your subject too contrasty and the colours washed out. If you want to get a soft, studio-like light effect get your subject out of direct sun but allow for controlled, directional light. A doorway, a tree or other natural shade is often ideal.
When you are in harsh sunlight consider using your flash to lift some of the shadow areas. But make sure that you consider why you're using the flash.