An important task for students interested in medical school is to use the summer and first year to reflect on why you wish to go to medical school and to get as much knowlege as possible about what that really means. If possible, talk to and shadow doctors, volunteer in hospitals, and so forth.
In terms of classes at Stanford, students interested in the health professions will typically consider taking chemistry and single-variable calculus in the first year. So you should certainly be evaluating and researching CHEM 31A/B/X-CHEM 33 and MATH 19-20-21. However, there are many paths to medical school, and depending on your desires, interests, preferred schools, background, and timeline, there is no single recipe for completing medical school requirements at Stanford. There is no pre-med major and medical schools are interested in applicants from all majors.
The School of Medicine offers a number of 1-unit lecture series that can deepen your understanding of the medical world, such as:
Many med school professors also teach IntroSems, Alternative Spring Breaks, and Sophomore College courses that can help you pursue an interest in medicine. Students also sometimes earn credit for volunteering at Cardinal Free Clinics--look out for information sessions when you come to campus.
Discuss your specific situation with your Academic Advising Director or AARC Advisor--all UAR advisors are able to give advice on course planning for first-year students interested in medical schoool.
In addition, UAR has several pre-professional advisors who have offered advice on planning for medical school on the web. Be sure to check out the Curricular Recommendations and download the Academic Planning Guide, which translates common pre-medical requirements into the courses offered at Stanford and discusses FAQs such as AP credit. You will definitely want to join the pre-med mailing list (eg 2022-pre-med) for your class year to hear about information sessions and other opportunities, and it is also a good idea to check out the Stanford Pre-Med Association and subscribe to their mailing list as well.