When placing a microphone, the primary objectives is to get the microphone as close to the subject as possible. When recording dialogue, this means placing the microphone close to the mouth without the microphone getting in the shot.
If you're using a lavalier microphone, clip the microphone 6 inches away from the subjects mouth on their clothing. If possible, hide the microphone behind a tie, button, or jacket. However, make sure there isn't any clothing rubbing against the capsule otherwise you'll introduce noise rustle which is very unpleasant to the listener.
If you're using a shotgun or cardioid microphone, place the microphone out of frame, either above or below, and positioned no more than 12 inches away from their mouth. As shotgun and cardioid microphones are directional, point the tip of the microphone to the chin or upper chest of your subject. This will capture the best sounding audio.