How can I force User to fill them out the content Profile?

there are two ways to do this, one is with rules and the other is with 'require' checkbox in fields. I would go with the former because you really don't want users to have to enter every single field in their content profile unless you really have to. Checkout rules you can set a rule that if when a user logins or registers and that user doesn't have a content profile type create, redirect to it or provide a reminder. furthermore in content profile options there is a checkbox that will remind users in their profile page to create their content profile.