The fitting will probably involve tests of your flexibility, various measurements, and then getting on either your own bike on a trainer or a "fit bike" so that he can see your form and how you pedal. If there's a computer, it will probably display your power output.

In addition to all of the body and power measurements, give him as much info as you can about how everything feels, and also what sort of cycling you intend to do. That can make a big difference in how your bike is set up.