Security is becoming an increasingly important topic, particularly now that more and more computers are connected to the Internet. This is a very complex subject, and one that deserves a book unto itself. Here we will provide an overview, but to fully understand all the issues involved, we strongly recommend you read Solaris Security by Peter Gregory.
The issues surrounding security will always be a compromise. The most secure server is in a locked, guarded room with no network connection, but then it may not be much use to anyone. Therefore, some of the points given below will be applicable to your circumstances, but not all. Bear this in mind when reading them.