What you wear for your headshot session will depend largely on your profession, how you'll use the photos, and where the photos are taken. In general, neutral colors work best (think gray, white, black). Pick how casual or formal your outfit is by what's generally appropriate to your profession. Make sure that your clothes fit, and aren't baggy or loose. Pick clothing that have vertically narrowing points, like a long necklace or a v-neck so that the viewer's eye is drawn up and down, not side to side. Style your hair and makeup to how you normally wear it, or hire a professional stylist if you want a more polished look for the photos. Lastly, don't wear something that you would never wear outside of the session. You'll be uncomfortable, and that will show in the photos.