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?
Drupal 8 is in its API completion phase, which means that after over two years of development, we're doing final work to finish the APIs that will ship with Drupal 8.0, including completing conversions, improving performance, and refining the developer experience.
- If you'd like to help core developers find bugs and improve the developer experience, now is a good time to start exploring Drupal 8, but there will still be some API changes that can break contributed modules between now and Drupal 8.0.
- If you want to avoid having to revise your module's code, you should instead wait for the first Drupal 8 release candidate before you begin your module upgrade.
Take a look at What Drupal 8 API changes are still outstanding? for some of the most important changes we're still planning to make. For more information, review the documentation on the Drupal 8 release cycle and see Dries' blog post, Drupal 8 APIs are freezing but not frozen.
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)