Moderation can be enabled globally or within specific applications. See How does moderation and abuse work? for more details about moderation and abuse.
To moderate all content created by a member, navigate to Administration > Membership > Members, search for the member, select the member, click Edit, and change the Moderation field on the Administration tab. Click Save. All content created or edited by the member will be entered into the moderation workflow.
Most applications support identifying that all content created/edited within them should be moderated. The options to enable moderation are specific to the application, but are generally located within the management options for the application.
Note that when an author is effectively moderated (either because of the account-level moderation setting or because the application into which the content is created is moderated), if the author can review moderated content where the content is being created, they are exempt from moderation.