setting comments unpublished by default for a content type?
If you don't want users to add comments without admin approval then you need to disable the below permission...

Skip comment approval

Once you disable it users would be able to post comments but they will not be visible to other users until the comment is approved by an admin.

This would set the permission for the entire contents and not for a specific content type.

If you want to set it for a particular content type then you would need to look into one of the contributed modules

commentaccess

Allow anonymous comments per node type

Comment Permissions - Stable Version only for 6 does not have a stable 7 version yet