First, in order to ensure the downloads are valid, add the GPG key for the official Docker repository to your system:

curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

Add the Docker repository to APT sources:

sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

Next, update the package database with the Docker packages from the newly added repo:

sudo apt-get update

Make sure you are about to install from the Docker repo instead of the default Ubuntu 16.04 repo:

apt-cache policy docker-ce

You should see output similar to the follow:Output of apt-cache policy docker-ce

docker-ce:
  Installed: (none)
  Candidate: 18.06.1~ce~3-0~ubuntu
  Version table:
     18.06.1~ce~3-0~ubuntu 500
        500 https://download.docker.com/linux/ubuntu xenial/stable amd64 Packages

Notice that docker-ce is not installed, but the candidate for installation is from the Docker repository for Ubuntu 16.04 (xenial).

Finally, install Docker:

sudo apt-get install -y docker-ce

Docker should now be installed, the daemon started, and the process enabled to start on boot. Check that it’s running:

sudo systemctl status docker

The output should be similar to the following, showing that the service is active and running:

Output● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2018-10-18 20:28:23 UTC; 35s ago
     Docs: https://docs.docker.com
 Main PID: 13412 (dockerd)
   CGroup: /system.slice/docker.service
           ├─13412 /usr/bin/dockerd -H fd://
           └─13421 docker-containerd --config /var/run/docker/containerd/containerd.toml

Installing Docker now gives you not just the Docker service (daemon) but also the docker command line utility, or the Docker client. We’ll explore how to use the docker command later in this tutorial.

With Docker installed and working, now’s the time to become familiar with the command line utility. Using docker consists of passing it a chain of options and commands followed by arguments. The syntax takes this form:

docker [option] [command] [arguments]

To view all available subcommands, type:

docker

As of Docker 18.06.1, the complete list of available subcommands includes:

Output
  attach      Attach local standard input, output, and error streams to a running container
  build       Build an image from a Dockerfile
  commit      Create a new image from a container's changes
  cp          Copy files/folders between a container and the local filesystem
  create      Create a new container
  diff        Inspect changes to files or directories on a container's filesystem
  events      Get real time events from the server
  exec        Run a command in a running container
  export      Export a container's filesystem as a tar archive
  history     Show the history of an image
  images      List images
  import      Import the contents from a tarball to create a filesystem image
  info        Display system-wide information
  inspect     Return low-level information on Docker objects
  kill        Kill one or more running containers
  load        Load an image from a tar archive or STDIN
  login       Log in to a Docker registry
  logout      Log out from a Docker registry
  logs        Fetch the logs of a container
  pause       Pause all processes within one or more containers
  port        List port mappings or a specific mapping for the container
  ps          List containers
  pull        Pull an image or a repository from a registry
  push        Push an image or a repository to a registry
  rename      Rename a container
  restart     Restart one or more containers
  rm          Remove one or more containers
  rmi         Remove one or more images
  run         Run a command in a new container
  save        Save one or more images to a tar archive (streamed to STDOUT by default)
  search      Search the Docker Hub for images
  start       Start one or more stopped containers
  stats       Display a live stream of container(s) resource usage statistics
  stop        Stop one or more running containers
  tag         Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE
  top         Display the running processes of a container
  unpause     Unpause all processes within one or more containers
  update      Update configuration of one or more containers
  version     Show the Docker version information
  wait        Block until one or more containers stop, then print their exit codes

To view the switches available to a specific command, type:

docker docker-subcommand --help

To view system-wide information about Docker, use:

docker info

You can read more details form digital ocean and docker’s official doc.