As soon as your new BSD Unix system is installed it is rather secure, although you may wish to take further steps to lock it down further. There are many techniques you can use to secure your BSD system. The most important step is to define a security policy. Once you have a clear plan as to how the system will be run you can then lay out your plan of implementation. Once you do have your security policy in place, make it available so that everyone who uses the system is aware of it.
Security is a vague concept.
As per the fine book from O'Reilly called Essential System Adminstation , security is defined as having the system remain in the state that you expect. If your system is doing things you do not expect, you can assume it is not secure even if you have not been hacked. If you have a user on the system who has root access but does not follow the security policy closely or does not keep you up to date as to what changes she is making, you will have problems. You want to ensure that you have control over the system. Your policy must be enforced and as the system administrator you must know what is happening on your system.