The structure of the Introduction can be thought of as an inverted triangle - the broadest part at the top representing the most general information and focusing down to the specific problem you studied. Organize the information to present the more general aspects of the topic early in the Introduction, then narrow toward the more specific information that provides context, finally arriving at your statement of purpose and rationale.
The Introduction answers the following questions:
What did I study? Why is this an important question? What did we know about it before I did this study?