Auto-focus works off of contrast in a scene. If you are trying to take a portrait then focus on the eyes not cheecks.
Focus is determined only by distance so you can focus on one thing and keep the shutter release pressed halfway and recompose without losing focus. This is a hugely important point.
Auto-focus is not half as bad as auto-exposure.
BTW, Auto-exposure is set as 'M' via your program dial on the camera and 'Auto\manual-focus' option is set via a small button generally to the left on the top side of your lens.
