Monitor Sessions
To monitor sessions in SQL developer, you may find the entry in the function menu of "Tools".

SQL Developer - Tools - Monitor Sessions
Select Connection
Then select which database you want to connect to and monitor.

SQL Developer - Choose Connection
SQL Developer Kill Session
Right-clicking on any one of sessions shows a menu, there're more actions you can do on it, which include killing the session.

SQL Developer - Kill Session
SQL Developer Reports
There're more various session monitorings in "Reports" panel. The navigation path is as below:
All Reports > Data Dictionary Reports > Database Administration > Sessions

SQL Developer - More Session Monitoring
You may take a look.