Collecting data for audit.

DB: 11g Enterprise EditionOS: Windows Server 2008

Parameter AUDIT_TRAIL: DB (since 2010)

With the goal of gathering data for audit, I need to collect the following data:

1) In the last year, which users have created or changed tables?

2) When the user SYSTEM was used and operations performed?

3) When the user SYS was used and operations performed?

And information:

A) How can I trace the actions performed by a particular user within a period.

Finally, how can I ensure that the data of audit tables were not deleted or changed?

Thanks