On the Liip blog there's a post from Jon Minder talking about the new Drupal 8 migration API/module and includes a guide to get you started using it in your Drupal application.

We at Liip AG believe, that the migration API is the best and most efficient way to import data into Drupal. [...] The Migrate 8 module in core is only an API. There is no user interface. This makes it difficult for new developer to start with Migrate 8.

I suggest you, to install the [several] extension modules right away before you start developing if you want to realize the full potential of migrate.

The post starts with a list of reasons they see to use the migration module over other methods including the fact that it's sponsored and supported by Acquia. He provides a list of modules to install before getting started with Migrate and links to a GitHub repository if you'd like a "quickstart" environment to follow his examples. He then compares the previous version (from Drupal 7) of the migration handling with the newer Drupal 8 functionality, most notably the change over to YAML configuration rather than PHP code. He helps you understand the structure of these files and gives an example. He shows how to run the migration, gives a more advanced JSON source example and mentions the included process plugin.