.. how far away from them do I need to stand in order for them to be in focus?
Today, I am shooting my first maternity shoot and want to make sure that her whole body is in focus... so would I use the focus points and JUST focus on her face or would I use the focus points that focus on all of her body??
Yes, I know you've already got several good responses, but if you're up for yet one more, here's mine:
The way I look at portraiture, I would not invade someone's personal space. We all have some tolerance for people getting all up in our face and such, and some of us have less tolerance than others. So for me, I would probably stay at least 8 feet away, and more if I had the space in which to back up. You choice of lens should accommodate how far away you are. For me, I would try to shoot someone's portrait (single) with somewhere between 75mm and 150mm. Maybe a small group of three persons I might go with 75mm to 100mm if I had the space available. This gets me far enough away from my subject so my subject feels comfortable about having a camera pointed at them.
The camera will focus at any distance.
Your camera will use only one focus point in actuality, even if several points are available. By selecting one active point, you are directing the camera's focus mechanism to use that one point and no other. I would probably make the one point fall on her eyes, assuming the most common point of focus for portraiture.
Now for the depth of field (DOF). This can be tricky to learn, and most of us do not memorize lots of DOF calculations. Rather, we have calculated the DOF enough times that we have a pretty good guess at what the DOF will be for most common situations. So what you can do is make some calculations (use the on-line calculator) for:
1. your lens
2. your approximate distance
3. your camera's sensor
4. various apertures that you pre-select in order to do the calculations.
That way, you have already got the DOF figured for several apertures, and away you go!