As of February 2019, the time of this writing, the current version of SQL Server is 2017. There are several editions of SQL Server 2017. If you just want to learn about SQL Server in a non-production environment, I recommend downloading the SQL Server 2017 Developer edition. You will also need to download and install the SQL Server Management Studio, which is only available in Windows platforms.

SQl Server 2019 Developer Edition

SQL Server 2019 Developer is a full-featured free edition, licensed for use as a development and test database in a non-production environment. You can download the 2019 Developer Edition here.

SQL Server Management Studio (SSMS)

SSMS is an integrated environment for managing any SQL infrastructure, from SQL Server to Azure SQL Database. SSMS provides tools to configure, monitor, and administer instances of SQL. Use SSMS to deploy, monitor, and upgrade the data-tier components used by your applications, as well as build queries and scripts. Use SQL Server Management Studio (SSMS) to query, design, and manage your databases and data warehouses, wherever they are – on your local computer, or in the cloud. As of the time of this writing, the current version is 17.9.1. From Microsoft Docs you can get a link to the download.