Getting the requirements
How do I know what my requirements are? Well, if the internal processes have been defined, this should be an easy question to answer. You should start by the most important and frequent processes and move on to the least important or infrequent processes. The important and frequent ones should be handled by the ERP while the least important and infrequent processes may or may not be handled by the ERP depending on your goals and the budget for the implementation.
Talk to the consultants who will be implementing Dynamics NAV for your organization. Tell them how your process looks like, who is involved in it, what information is required, and so on. They will tell you how this specific process is resolved in Dynamics NAV.
If the way Dynamics NAV handles the process meets the way you handle the process, you've struck gold! You will be able to keep on doing what you're already doing without any kind of modification in the behavior of the application. This is...