A major decision for me was majoring in Computer Science, I always had an interest in biology and medicine but knew that CS could be leveraged to enhance these fields even further. Also choosing to focus on research vs. software engineering internships in undergrad really gave me a strong research foundation that helped me in my successful admission to PhD programs!
As for advice, if you want to do a PhD, I would suggest you start looking into research opportunities or programs on campus as soon as possible or look into AI residency programs. It's hard to admit students into PhD programs without research experience and I find that many students put this off until later.