Delete everything except a moving object

tracing over a live video is called Rotoscoping. here's an old but still good article about it: https://www.fxguide.com/featured/the-art-of-roto-2011/

there are several ways to approach this:

1. Masks - Managing and animating shape paths and masks in After Effects

2. Rotobrush tool - After Effects Roto Brush, Refine Edge, and Refine Matte

3. Mocha Ae - it is bundled with your Ae release since CS4 - it's main use is for tracking, but also roto masking. after you roto your element, you can bring the information back to Ae as masks or mocha shape effect : Paste mocha roto shapes to After Effects masks on Vimeo

one cool technique to help better trace your moving object is to use Reverse Stabilization. you stabilize your object, trace it with as minimum keyframes as possible, then re-introduce the motion. here's an example by our community's guru Rick Gerard: Simple Roto - YouTube

you can also show us the footage so we can better assist you by giving specific instructions.

any ideas guys?