You are eligible to declare a major starting after the add/drop period in the semester you reach second-semester sophomore standing. The best time to declare is between the end of add/drop and the start of course pre-registration for the following semester.
Explore this site to learn more about the major declaration process, including resources that can help you with this important academic milestone.
My plan was to eventually become a high school English teacher, and I was afraid that majoring in religion (which I was going to do solely, at first) would prevent me from doing so. I even called the Teachers College at Columbia University, and spoke to an Admissions officer there about my concern. He reassured me that it does not make a difference, and I realized that I should make the most of my time at Wesleyan by studying out of interest, not out of need. That's what I did, and I was extremely happy as a religion and English double major.
--Lily