Course selection is a balance between pursuing your academic interests and exploring new academic areas.
Information on the Harvard College Curriculum including the General Education, Divisional Distribution, Quantitative Reasoning with Data, Language and Expository Writing requirements can be found in the Harvard College Handbook for Students (https://handbook.fas.harvard.edu/book/welcome). Specific information about each concentration can be found in the section of Handbook called Fields of Concentration.
You can also review your Academic Advising Report (AAR) in my.harvard for updated information on your degree requirements. The AAR contains both Harvard College Curriculum and concentration requirements.
The primary source for searching for courses is my.harvard Course Search (courses.my.harvard.edu). You may also use the Syllabus Explorer https://syllabus.harvard.edu/ which allows you to search from a wider range of information about the course as well as Curricle https://curricle.berkman.harvard.edu/ which allows you to search courses in unexpected ways.
You are always encouraged to reach out to academic advisers for guidance in selecting courses. Both your assigned advisers as well as advisers in each concentration (generally Directors of Undergraduate Studies or Assistant Directors of Undergraduate Studies) are available at any time during the year to discuss ways to explore your academic interests through courses.