A lot of people have different views on what a good portrait lens is, what focal length, prime or telefoto. I don't think there's any "best" portrait lens. I think it all comes down to what perspective you prefer and what kind of capabilities you need (if you need wide aperture or a fast AF). You gotta think about all of that stuff and then do some research by reading reviews and looking at places like pbase to get an idea of image quality and everything - Google is your friend. Some traditional portrait focal lengths are 50mm, 85mm and 135mm.
Edit: Also, you should just get out there and scout for a location. I suggest you go out at the time of night or evening that you want to do the portraits so that you can see things as they will be for the shoot.