So you want to give your drone object tracking and following capability? shashankvkt8 has shared a guide that shows how to use a camera mounted on a 3-axis gimbal to track objects. The moving target is always kept at the center of the frame while the camera tracks it.
You are going to need a Pixhawk PX4, Odroid XU4, F450 quadrotor, gimbal, and Sj4000 Camera to get started. More information is available here.