To create and export your project with a transparent background, follow this guide:
1. Create a new composition and add your video or image to the composition
2. Add a solid layer to the composition by going to Layer > New > Solid
3. Use the Pen tool to draw a shape around the area of the video that you want to make transparent
4. Go to your footage and change its Track Matte to Alpha Matte or Alpha Inverted Matte depending on what area you would like to hide.