SharePoint Designer 2013 is a great tool for defining External Content Types. Its graphical driven no-code approach makes if fairly easy to expose external data. Visual Studio is another tool that can be used to define External Content Types and has an advantage with more complex data configurations but it requires developer skills to create.
The following walk-through will show you how to use SharePoint Designer 2013 to create an External Content Type mapped to the Employees table from the Northwind SQL database.
You will need access to the Northwind database stored on a Microsoft SQL server for this walk-through. Additionally, you will need metadata store permissions to the Business Connectivity service in your farm.