First, you want the Focus Mode (front of camera near lens) set to Continuous not single or manual. This allows the camera to keep following the subject once the system has "locked onto" the subject. You want Auto on the lens switch as well.
Then you want the Auto Focus Settings (rear of camera near the Multi-Selector) set to Auto Area. This will let the camera use the entire central portion of the frame and all the focus points to try and acquire (and keep) the the subject...which is hard to do! This makes it easier.
Note the picture shows Single point selected not the Auto Area.
Finally, the Release Mode, should be on CL or CH (Continuous Low or Continuous High) (on top left of the camera) to allow for quick exposures. The CH can be as high as 11 frames a second.
Great Blue Heron