I am new in OEM so please tell me, Which type of alert should i set to understand Database Connection Timed out in OEM 12c.
The Database Connection Timed out error are logged in the DB alert.log
You can set the Setting thresholds for alert log metrics in Cloud Control 12c to get alert for the same
Refer to below metalink doc for details
Database Alert log monitoring in 12c explained (Doc ID 1538482.1)
Also you can refer to below doc in case you want to generate a report for these errors
How to create a Customized Report in 12c Cloud Control for Database Alert log Errors (Doc ID 1528268.1)