Microsoft Announced that Windows Server 2019 is now Generally Available, they showed up new Features and improvements at Microsoft Ignite.
The new release is designed and engineered to help modernize your datacenter, delivering on four key areas Hybrid Cloud, Security, Application Platform, and Hyper-converged infrastructure. Read More.

In Windows Server Documentation you can read more and go deeper on the details.
You can check the new Upgrade Center, where you can find useful information for the upgrade process.

Download Windows Server 2019:
Volume Licensing Service Center (VLSC).
Evaluation Center.
Azure Marketplace.
Later in October, will be adding Windows Server 2019 to Visual Studio Subscription (former MSDN) and other portals, such as the Microsoft Partner Network (MPN).

If you are looking ahead, join the Insiders program. Microsoft will continue to ship new builds of Windows Server that will first land on the next Semi-Annual Channel and later in the next Long-Term Servicing Channel.