This handbook provides information and specific steps for upgrading contributed and custom modules from Drupal 7 to Drupal 8. Drupal Module Upgrader can do some of this automatically for you, covering many API hooks.
Should I start upgrading now?
Likely information to apply: a large list of Drupal 8 module and site development resources and videos at drupalize.me. PSR-4 autoloader patterns are used in Drupal 8.
See also converting themes to Drupal 8: (as it's likely you will need theme features as well as module API changes)