Martin Burns has a great post on project requirements:

Step 1: Context Is Everything

Why did I mention Goals above? Because if you don't understand these, you won't get to the Objectives (and therefore Requirements) that the Sponsor actually needs. Take the Moon Landing project. The Goal was "Convince the World of the Superiority of American Technology." Just landing on the Moon wasn't enough - the Project also had to convince the World that it had happened (leaving aside Conspiracy Theorists for now). So a key requirement of the project was to ensure global publicity, including live television broadcast.

So step 1 is really Understand the Goals. How do you do that? With luck, it'll be documented for you (and if it's not, the old PM rule of If it's not documented, it's a rumour holds true), but unless you enjoy working on assumptions, you talk to the Sponsor and other key stakeholders (ie people who have an interest in the outcome of the project). Useful questions to ask include:

• Why are we doing this?
  
• If we're successful, what would be the outcome for you?
  
• What would happen if we didn't do this project?

Note that these are all Open questions designed to get the other person talking...

...via Cutting Through