In order to get help with Docker you can use the following
commands:

docker --help
docker help run
docker help ps
docker help images
docker help rm
docker help rmi

If you type in on the command line:

docker --help

then you will see a list of the command available for Docker. You might need to scroll-up to see the beginning of the output
or on Linux and OSX you could run it through less

docker --help 2| less

You can ask for help by typing docker help and then the name of one of the commands so docker help rmi for example
and then it gives you some kind of an explanation of what does the rmi command do.

You will find a lot more detailed documentation on the web site of the documentation of Docker.
There you can search using the name of the command and find all kinds of documents.
For example the reference page if docker rmi.

That's basically how to get help, though you can just search on your favorite search engine.