You will find that there are many more social sciences fields and possibilities at Stanford than there were in high school, including Anthropology, Communication, Economics, International Relations, Political Science, Psychology, and Sociology. In addition to Stanford's own social science departments, many interdisciplinary programs also draw heavily on the social sciences.
If you are looking at this before New Student Orientation (NSO), you should consider attending a relevant Social Science Academic Planning Session during NSO.
There are dozens of courses that may be a good starting point in the social sciences, depending on your particular interests. IntroSems are a great way to leap right into a field, and Cardinal Compass maintains a list of additional courses that departments think are suitable for frosh. You might use a keyword search in ExploreCourses, or browse the prefixes for the departments mentioned above (e.g. ANTHRO, COMM, ECON, INTNLREL, POLISCI, PSYCH, SOC). Keep in mind that intro courses will typically have lower numbers and appear toward the beginning of the course listings.
If you're interested in Economics (or related disciplines such as Public Policy or International Relations), you should consider taking ECON 1 in your first year. You should also think about getting started with Math. ECON 50 is a key course that in most cases should be completed by the end of your sophomore year. It requires both ECON 1 and MATH 51 (or CME 100). If you have placed into MATH 51, you can take it Autumn, Winter, or Spring. If you are starting with calculus, then you should take MATH 19 in Autumn Quarter; otherwise it will be difficult to get through MATH 51 and ECON 50 by the end of your sophomore year. (MATH 19-20-21 is the three-quarter calculus sequence.) Please note that the Math department requires students to take a placement diagnostic in order to enroll in any of the introductory Math courses 19 through 51. The placement diagnostic is taken on-line and students can take it anytime during the academic year. For more information, please see http://mathematics.stanford.edu/academics/undergraduate/math-placement.
Aside from courses, there are many other opportunities for students interested in the Social Sciences. Consider exploring programs offered by the Haas Center for Public Service. Many of the programs offered by the Haas may appeal to students interested in the social sciences. Pursue research with Stanford faculty--browsing the list of VPUE-funded research programs reveals a number of social science departments, and many research centers will be using social science methods. Sophomores pursuing in-depth projects in the humanities, creative arts, and qualitative social sciences are eligible to apply for Chappell Lougee Scholarships: if the idea of spending a summer on your own project intrigues you, begin thinking about ideas and talking to your Academic Advising Director or AARC Advisor about the Chappell Lougee during your first year.
For more information, check out our introductory guide on how to get involved in the Arts, Humanities, and Social Sciences.