Per each content type, "create" permission can be assigned to certain roles. For example, the content type of "wiki" can be created by authenticated users (signed-in users) The content type of "poll" can be created by only content administrators.

1. Goto "Administrator -> User management -> permissions".
2. Look for the specific content types (e.g. wiki) and check the box for those roles that you want to/not to have the "create" permission.