IMHO the official Docker documentation is great! Their "getting started" page is a good overview of the Docker basics. https://docs.docker.com/get-started/ The CLI --help is pretty useful too. I have also found Docker's in-depth documentation to be very helpful when trying to figure out the more complex features.
I got started by just grabbing a $5 DigitalOcean droplet (can get them with Docker pre-installed) and then played around trying to setup a simple app. (I think it was RocketChat.)
https://rocket.chat/docs/installation/docker-containers/
