How to Use Camera Aperture to Narrow Depth-of-Field
Using the camera aperture setting on your camera, you can capture different focal points with your camera. The aperture determines where your camera sensor focuses on and where it takes its measurements from. Larger aperture (using a smaller f-stop) and closer subjects allow for a shallower depth of field, while the reserves is true for images farther away. Focal depth (measured in mm) can give you a half a meter in distance on film. There are plenty of conversion charts out on the Internet that will help you do the math for this.
Auto Focus Feature
Most cameras will have an auto focus feature; this allows you to set where you want the focus as your snapping the picture. You should see a tiny rectangle on the viewing window. Line that up with your subject and snap the picture. Alternately, adding a light measuring device can also improve your chances of a better shot and setting the device manually.