Get an overview of the application from an end user's perspective
Before you start attempting to understand the technical aspects of an application, talk to end users. You want to get a high level summary of the application, aiming to find out the following as an absolute minimum:
- What does it do
- Who uses it
- How do existing users run it (everything from a desktop or start menu shortcut to a file on a network drive, through to corporate management tools, click a URL from a web page or manual installation etc).
If there are people who are responsible for managing and supporting it, talk to them about what it does and how they manage it, and any problems they frequently have to deal with. This could be helpful in understanding aspects of it (eg broken profiles resulting in lost ODBC settings meaning database connectivity doesn't work, or automatic upgrades of runtimes moving applications to a version that is incompatible with the software).