TwoPynts is right - just talk to them in a normal way. Get to know them, get them comfortable with you - once you can establish a good rapport the shoot will go pretty well.

Something to note - when photographing women for fashion (or whatever, frankly - a woman model), don't "touch" them. Gesture with your body for them to copy - put your hand back here, tilt your head back a little, etc. Make sure when hiring a female model that you have a female assistant somewhere in the room to help if their clothing needs readjusting, hair needs to be set, etc. Be respectful, and realize that you touching them could easily be mistaken for copping a cheap feel, or coming onto them.

Don't be nervous, and they won't be either.
Hope that helps-
-Lew ;-)