I assume you are using the contact starter solution, but the is a guess since you didn't state. If so, the contact db used the Contacts::Contact Name Labels field. You can changes this in a couple of different ways. The first way you could just modify the calculation in the above mention field, or you can modify the button that you click to send the email. right click the email button, click button setup, click specify, then change the Contacts::Contact Name Labels field to Contacts::First.