Lets create a docker-machine first. Here I have named my machine docker-ubuntu with 2 GB RAM and 2 CPUs.
$ docker-machine.exe create -d virtualbox --virtualbox-memory "2048" --virtualbox-cpu-count "2" docker-ubuntu
Running pre-create checks...
Creating machine...
(docker-ubuntu) Copying C:\Users\Animesh\.docker\machine\cache\boot2docker.iso to C:\Users\Animesh\.docker\machine\machines\docker-ubuntu\boot2docker.iso...
(docker-ubuntu) Creating VirtualBox VM...
(docker-ubuntu) Creating SSH key...
(docker-ubuntu) Starting the VM...
(docker-ubuntu) Check network to re-create if needed...