Before you can send your first SQL statement to the database, you need to create a connection to the database server. A connection consists of a username, password, and connection string or hostname. This connection authenticates you to log in to the Oracle database.
When you first evoke SQL Developer, you will see screen similar to below Figure. The screen is divided into several panes. The left pane, labeled Connections, allows for a list of database connections.
Step1: The Connections window in SQL Developer
Step2: Creating a new database connection
You can add a new connection name, such as connection, and assign a username as development and password. For the purposes of the examples in this tutorial, use the username development and the password.
Step3: New / Select Database Connection dialog box
When you click the Save button, you see the connection name added to the Connections window, as shown below
Step4: List of connections
When you double-click the connection name, you are connected to the database, using the appropriate user account and password. If you did not check the Save Password box when you created Connection, you are prompted for it each time you open the connection.
You can modify the connection information by right-clicking on the Connection node and choosing Properties from the context menu, after performing above steps you will be able to connect the database of Oracle application release 12.