What is the first step in computer security?

The first step in securing a network is knowing what you have; you need to conduct a security audit of the environment. In order to secure and protect a network you need to know what is in it, then find the risks with those areas, and the vulnerabilities associated with the risks. Then you assign the probabilities of being exploited via those weaknesses and decide on how to mitigate the risk (or eliminate it, if possible).

But the first step is always the information audit.