Good starting point for learning Docker is to read following sections from the official Docker documentation. It is mainly a series of short tutorials, together with related documentation about syntax of commands, etc..
- Install Docker Engine
- Verify your installation
- Learn about images & containers
- Find and run the image from Docker Hub
- Build your own image by extending existing Docker container image with new
Those are focusing mainly on how to get existing things from Docker Hub. Next thing to learn would be how to create things for running custom software on top of base Ubuntu Linux system image.