I've done some previous articles on this topic. I recommend you start with part one of a three-part series on performance testing called "Testing for performance." In that article I talk through the steps I take to develop an initial workload model and I reference Scott Barber's work on the User Community Modeling Language (UCML).
In an article on Analyzing Performance-Testing Results to Correlate Performance Plateaus and Stress Areas I show an example of how you can work to correlate your testing results back to your model. Finally, take a look at the chapter on workload modeling in Performance Testing Guidance for Web Applications. You can also find the material online, but if you're new to performance testing you might want the book - it's likely the best place for you to start for most of your questions.Good luck!
This was first published in July 2009