Make sure auditing is enabled by setting the initialization parameter AUDIT_TRAIL. Changing this parameter requires bouncing the database.
To audit activities on a specific object, use a command similar to "AUDIT ALL ON <object_name> BY ACCESS". the "ALL" option can be replaced by specific actions if you only want to audit a subset of those actions. For instance, auditing select activity on a heavily used table will generate lot's of audit trail data. This may impact performance.