A second backup job (in addition to the full backup) is needed if you intend on making differential backups or transaction log backups.
Have a look @ the best practices for agent for sql as well - http://www.symantec.com/business/support/index?page=content&id=HOWTO74429
You just need to create a new job and select the SQL databases to be backed up through the Microsoft SQL Instance node in the job selection list. You can also add the same in your daily backup.
http://www.symantec.com/docs/HOWTO74429 : Best practices for Backup Exec 2012 Agent for Microsoft SQL Server
The Admin Guide is going to have all you need to know around setting up and running backups of SQL.
Here's the Best Practices too: