What is required to study economics in harvard?
There are no strict entry requirements (for coursework or achievement thereof) from high school to study nearly any discipline at Harvard or equivalent universities in the US (as there are for courses in the UK).
In fact, when a student applies for an undergraduate program at Harvard via the Common App, you indicate interest in just one of the following “academic programs”: Biological Science, Computer Science, Engineering, Humanities, Mathematics, Physical Sciences, Social Sciences. This is in contrast to some other universities that ask applicants to indicate a specific concentration (re: Economics or some variant thereof). The rationale for this is that over the course of your General Education studies at Harvard, you’ll exposure yourself to interesting disciplines that may shape your Concentration.
Getting to the core of your question: First and foremost you should demonstrate an intellectual interest in economics (most readily via coursework at school and/or via outside classes) and, more importantly, have a view on which critical issue or issues of today or the near future you’d like to resolve w/training in economics. This second component is more difficult and will require a fair bit of outside reading and leadership in local and global problems while in high school. Beyond stellar grades and standardized test scores required of all Harvard students, your extracurricular engagement and reflection thereof will set you apart.
I would encourage you to reach out to Jaime Beaton (email@example.com) with further details on your specific context. As an Applied Math-Econ alumnus of Harvard, he may provide insight on how you can strategize for the remainder of your high school career.