This is an introduction to Step 1, Categorization of the NIST SP 800-37, Risk Management Framework process. Categorization consists of three primary steps:
1) Determining the Security Categorization of the information system. This is done by breaking down the primary information types on the system. You can get great guidance on this from FIPS 199 and NIST SP 800-60 (Volume I-II).
2) Create a System Description. This is really the first step to creating a System Security Plan and it leads to registering the systems.
3) Register the system. This means that you need to advertise the the system to all the stakeholders of the system in the organization. Organizations usually have a method of doing this with a database that can be seen by upper-level management.