With most modern cameras, focus on subjects is straightforward and in most cases the camera does a pretty good job. How sharp the focus is depending upon factors such as the lens that you are using and conditions under which you are shooting.
In most traditional DSLR cameras, the photographer will see a grid similar to what is displayed above. Some cameras have many more points of focus, but for simplicity sake I have chosen the example above. The camera will try to keep in focus the points as shown through the viewfinder. It can be fooled by situations like moving objects or lack of contrast in a subject matter. In the picture of the model above, she is sitting stationary. The water of course is flowing past her, bouncing off her in many directions. This is a case where the image may not be as sharp as one would hope.
One way of dealing with this would be to zoom into the model, focus on her, re-compose, and then shoot the image with the focus locked by holding down the shutter button half way, or using a back focus function (discussed in an earlier post). The issue here to be honest was that the water was [COLD]. A cold model is probably not going to give you her best effort if you let her stay cold for very long.
Most cameras will allow you to select ONE focus point. Although the center point is generally the most accurate, if you select another point, you are telling the computer in your camera that you want to make that point extremely important. How you choose that point will vary from camera to camera, so one would need to consult your camera manual. Once you have selected that point, place that single point over the model, and shoot the image(s). Typically you will find that your shots will be much sharper that you may have otherwise produced.
A side effect of this, in some cameras is that you will need to adjust your exposure. In the model above, she is darker than the light hitting the water so the correct exposure for her would be significantly different than for the water itself. Consider the 'autobracket' feature of your camera. The autobracket feature will allow you to take multiple exposures fairly quickly while underexposing, correctly exposing, and over-exposing your subject. How you set this feature is typically specific to your camera model, so once again you will have to check your manual. You can often select how much or how little change in exposure you want to make for each shot. It can range from 1/4 stop to a full stop for each consecutive image.
These techniques may save you editing time in post production and allow for a overall sharper image.