Preview 5 of .NET Core 3.0 has been released which includes a new Json serializer, support for the publication of single executable files, an update to the runtime roll forward and BCL modifications.
Other new features include improved startup performance for WPF and Windows Forms. WPF and Windows Form assemblies are now compiled in advance, with crossgen.
The download is available from the site: dotnet.microsoft.com
If you want to learn how to use .NET Core I suggest you read Programming ASP.NET Core.