I'm a photographer myself with a little comercial experience.You can see a -kinda outdated- sample of my port at http://www.modelmayhem.com/740879
For someone starting out this would be my pointers:
- Practice some poses in front of the mirror, get someone to snap some photos (even with a cell phone) to see how those poses look on you.
- I'll say it again, learn a handful of poses. I've dealt with models that only have a single automatic pose. Like those people that have the same pose on their facebook albums. Really irritating.
- Don't worry too much about the clothing. Just make sure it is on good condition and without any big logo or brand. Do make sure it fits you. There are trick the photographer can do to improve the fitting of loose clothings, but there shouldn't be the need for that.
- Mix it up, sport, casual, even underwear if you are confident enough with your body -I've seen your profile here, you should be more than confident.
- Ask the photographer/agency beforehand about shaving and haircut.
- Relax, the first shots are most likely to be awful, then things kind of follow into rhythm.
- Mind your step, some of us tend to loose the nerves with models knocking over lighting stands and unnecessary stepping on the backgrounds.
- DON'T DO ANY TYPE OF PICTURE you feel uncomfortable doing (like impromptu nudes, for example).
- Your face just ask for a natural light portrait. Don't be afraid to ask -asking isn't the same as demanding- the photographer to highlight your eyes with the lights-n-shadows.
- Read the model release form and keep a copy for yourself.
- Get some sleep the night/days before, no one likes spending hours in photoshop fixing red eye-whites and puffy eyes.
- This one doesn't really apply to you, but if you got skin problems, cleaning up your diet -in with the fruits, out with the soda/spicy/junk food the week before really does makes wonders for your skin. Same goes for diet/nutrition supplements that you know cause you acne, stop them till after the shoot.