Black and white portraiture is my favorite type of work. (I’m a budding freelancer, studying hard. :))
Is your friend going to use C-41 or real black and white film? (C-41 b/w is film that looks b/w but is processed like color.) I’d recommend real b/w for the best results, although C-41 is a hell of a lot cheaper to process. (I like to use it sometimes as proofs, and then take the good prints to a good custom lab and have them printed on b/w paper.)
C-41 b/w always has a color cast, whether it be brown/blue/green, so you won’t get the beautiful pure tones and contrast you’d get out of real b/w.
Anyway, as for my advice for you…PLEASE do not worry about how you look. Your friend will probably take many pictures, so don’t stress over each one. Let her know which side you’re more comfortable with, but beyond that, relax and let her do the work. Nothing’s more difficult than a subject who’s trying to “see” the picture while posing for it. The most beautiful portraits are taken when the subject forgets the camera is looking; most people tense up and keep trying to force their “picture smile” onto their faces, or worry about the angle of their chins, or whether “this makes me look fat” or “are my smile lines showing” or whatever. Vanity, oddly enough, has ruined many potentially great pictures.
I’m sure you have a beautiful smile; relax and let it shine. Don’t hold the same pose forever; do what feels natural to you, make faces, laugh out loud, look serious…give your photographer a range to work with and I promise you’ll get wonderful portraits of yourself.
As for what to wear…honestly, IME, people look best in whatever they’re most comfortable in. I did a shoot of my gal pal and her boyfriend, in which she wore a simple black and white sundress with a big flower print…she’s normally a jeans type of gal but she loved that dress because it made her feel feminine, and she lit up the photographs while wearing it. The print wasn’t a problem at all.
And I’m extremely pale myself, and one of my favorite pictures of myself is one in which I’m wearing a simple white T-shirt and a baseball cap.
My point is that there aren’t many rules which aren’t made to be broken. Wear what you like, what flatters you, what you feel good wearing. If it happens to be a white T-shirt, wear it! No one is so white that their skin will match white fabric, and white is also a great light-reflector, so you may want to consider it. I personally don’t like wearing dark/black clothing in b/w, because it makes me look even more pale. Which may be the dramatic emphasis you’d like…a lot depends on what kind of pictures you’re doing, and what the photographer’s goals are…
So my main advice for you is to R-E-L-A-X. A good shoot is one in which you get a handful of pictures you really love; I’ve never seen anyone who can shoot four/five rolls of film and love each freakin’ picture. So don’t worry about each shot; just take a deep breath and go with it.