In order to start security auditing within Active Directory:
Log in to Windows Server as an administrator.
Go to Start, click on Administrative tools, and select Group policy management console.
Get to the domain/OU to be audited.
Right-click on the Group Policy Object, and choose Edit. This will open the Group Policy Management Editor.
In the left-hand tree menu, expand Computer Configuration, then Policies, expand Windows Settings, then Security Settings, and finally Local Policies. Click on Audit Policies.
In the main panel of the Editor, click on Audit object access and select both the Success and Failure options.
Click on Audit directory service access and select both the Success and Failure options.