If you're using a handheld microphone, you usually should hold it about 5-6 inches below the interview subject's mouth.
Don't hold the mic right in front of the person's mouth, but slightly off to the side and tilted toward the mouth. This will help avoid picking up "popping" noises from a person's lips as he/she speaks.
Tell the interview subject to try to ignore the mic and concentrate on the camera.
If it's noisy, then use a lavalier clip-on microphone to reduce the ambient sound.
But watch for necklaces or chains on a person's neck, or buttons on a shirt, that could rub against the lav mic and create noise.
With a lav mic, you'll need to "dress the mic" - properly attach it to the person you're interviewing
Ask the person to run the cord to the lav mic up the inside of their shirt (so the wire won't show in your video).
Then clip the mic to the outside of their shirt, about 5-6 inches below their mouth. Try to center the mic as much as possible. If you have it too far to one side, it won't pick up the audio well if the person then tilts his/her head to the other side while talking.
Use this same procedure if the person is wearing a t-shirt, running the cord up under the shirt and clipping the mic near the top of the shirt.
If the person has a necktie, run the wire down the back of the necktie and through the little label on the bottom back of the necktie.
If it's windy, the lav mic will pick up the sound of the wind. In this case try to clip the mic closer to the person's mouth, or switch to a hand-held microphone with a windscreen on it that muffles the noise of the wind.