Depending on your process—Agile, or more traditional—you will determine your needs in a less or more formal way. Whatever your process, some approaches are always appropriate.
First of all, it's useful to review the functionality, data structures, and data in your existing systems. Functionality is the easiest to discuss. Define the data fields and collection forms in your current system—custom data and profiles in CiviCRM's terminology—that may be needed in the new system. As you analyze existing systems, don't be limited to your primary contact database alone. For example, if you use paper sign-up sheets at events or have other paper, e-mail, or web-based data collection forms that are not part of your primary system, be sure to include them in the discovery analysis. If you use third-party web-based tools for broadcast emails or advocacy-related engagement, consider how those datasets will by migrated into or integrate with your CiviCRM data. Begin listing, in detail...
