Write out your research question.
What other questions would you need to answer?
Now, how do you use these questions to generate search terms? Identify the primary nouns in each question - those are your main keywords. Then, make a list of possible related terms and subtopics. Combine these terms using AND and OR (see the Tips & Tricks for Effective Research) to form search strings.
Global Trade Resources
Social, Political, and Historical Resources
These are just some of the resources the library has (using the search string "Spain AND Internation Trade") - Add a time period or specific factor to your search for more specific resources.