You need to understand what you mean by "security" before you can go about the task of securing a computer system. Traditionally, information security has meant ensuring confidentiality, data integrity, availability, consistency, control, and audit. But the relative importance of these items will be different for different organizations.
One way to grapple with these differences is to perform a detailed assessment of the risks that your organization faces, the impact that each risk could have, and the cost of defending against each risk. This is a long and involved process that few organizations are prepared to execute properly. For this reason, many organizations outsource their computer security work?the policy formation, the monitoring, or even the implementation. Other organizations adopt industry "best practices" and hope for the best.
No matter what you do, it's best if your decisions are informed by conscious policy choices, rather than by inertia, inattention, or incompetence.