As this page points out, the bare bones of what you need to test a migration are: Once you have these basics in place you can proceed with a migration from either the UI or using Drush.
Drupal 8 is still in the release candidate phase (as of the time of writing this post), and there are a list of known issues with the Drupal-Drupal migrate path as well that will likely persist into 8.0.0.
Editor's note: There have been some changes to Migrate in core as of Drupal 8.1.0 and Migrate Plus is now joined by Migrate Tools.
To learn more about more recent migration practices read Custom Drupal-to-Drupal Migrations with Migrate Tools.
As for the migration paths, this is one place the Migrate UI comes in really handy, as you can see in this screenshot: To get started, on my local Drupal 8 site I've enabled the Migrate, Migrate Drupal and Drupal Upgrade modules.
With these modules enabled, we can now visit the upgrade form (at the path /upgrade).
Most problems you'll run into attempting a migration right now will fall into one of these two categories: Before actually starting a migration, it's probably a good idea to identify the contributed modules you're using on your site and see which ones have Drupal 8 versions already available to test.
Indeed, the first attempt lead to a batch failure during the user import process.