Some people can be a little un-easy about getting their picture taken by a photographer. There are a variety of personal reasons one could feel that way. I am here to tell you that it doesn't have to be, nor should it be a bad experience. A good photographer understands that their primary responsibility is to make this a great experience - even more important than the final product delivered to the client.
Trust is very important aspect of a photo session. You and the photographer should have the same goals and be comfortable with each other. Hopefully this is a relationship that will last well beyond your session.
Once in the studio or on-location you shouldn't have to worry about much. This is where the photographer typically leads the effort of working through different set ups, lighting, posing, etc. But, this is a collaboration so don't be hesitant to be a participant in the creative process. I love it when a client gets into the event and suggest some interesting ideas. To help ensure a great experience there are some things you can do ahead of time to minimize any stress. Just some basic things to think about and do before a photo shoot.
Communication
Rest up
Pick out your wardrobe in advance
Bring what you need
Allow more time than expected
Relax