To migrate from Magento 1.x to Magento 2.0 you need to port all of your data and upgrade your extensions, themes and customizations to take advantage of new development approaches and technologies that offer a much greater ability to customize and scale your site.
We created the Magento 2.0 Data Migration Tool to help you efficiently port all product, customer, and order data, store configurations, promotions and more to Magento 2.0: https://github.com/magento/data-migration-tool-ce
A white paper describing the tool and the data migration process is available online: https://magento.com/resources/magento-2-migration-whitepaper
Magento 2 Trained Solution Partners can help you update your themes and customizations and documentation is also available online at devdocs.magento.com.
Code migration tools, which are helpful when you need to upgrade a large number of customizations, are available on GitHub: https://github.com/magento/code-migration
Magento has been actively working with the developer community to ensure widely-used extensions are updated and available for the new platform.