If you are new to the business of modeling, I would urge to to spend some time reading about modeling with STELLA so that you understand a little about how the program works, but more importantly, so that you understand the uses and limitations of models, and the various purposes of modeling. Modeling is ultimately of little use unless you can answer the following question: What do the model results mean?
For those of you who are rushed for time, there is a shortened and slightly different introduction that can be found here.
2.1. Introduction and What is a Dynamic System?
2.3. First Example: The Bathtub
2.5. Departures from the Steady State
2.7. Response Time vs Half-Life
2.8. Response Time vs Residence Time
2.11. Common System Structures and Behaviors
2.12. Purpose and Meaning of Models
2.13. How Simple is Simple Enough?
2.14. Process of constructing Models
2.15. Learning Through Experimentation