To require or unrequire validation of items, go to Admin Panel -> Members -> Manage Usergroups. Select a group, check check or uncheck the boxes for requiring validation for topics, forums, posts, etc. Member accounts are a special case: to require validation of member accounts, go to Admin Panel -> Members -> Member Settings -> "Registration Type:" and select "Require manual validation by admin" (or another option as best fits your desire).

To review currently pending items, go to Admin Panel -> Common Tasks -> Pending Validation. This section contains tabs for each different type of item which is pending. You can approve, reject or edit each item as desired. You may provide a reason which will be emailed to the submitter in a message telling them their submission has been approved/rejected, or not.

Note that emails are automatically sent to the submitter on approval or rejection, unless you uncheck the notify submitter of decision box. To change the text of these emails, go to Admin Panel -> Customizations -> Language. These are the names of the applicable language items: email_registrationdeclinedbody, email_comdeclinedbody, email_rejectbody, email_rejecteditbody.