Understanding the data and business you are in is essential to building a good data model. You cannot have a working and performing model unless you know in detail not only what your data means, but how you can connect it so it makes sense.
Grab a piece of paper and start writing down what you know about your data. What are you trying to communicate? What are the most relevant fields in your data source based on your requirements? How do they relate to each other? These are pointers for you to step back and think about what you want to achieve before you actually get your hands on the data.
You can even load files or tables into an application and sense the different fields and formats, and start thinking about how to reduce your data and fields.
A star schema is a logical representation of your business into facts tables and dimensions....
