To start building a schedule, it’s helpful to think about each class as part of a strong foundation. You want your foundation to support future work, but you also want some breadth so that the structure you build feels more like a comfortable house than a grain silo. Consider your long range plans, but allow yourself an opportunity to explore. In this way you will develop a personally coherent, meaningful education out of the diverse courses that Stanford offers.
Figure out what fields, topics, approaches and faculty appeal to you personally. Don't rely on the experience of others.
Explore unfamiliar subjects. Many of Stanford’s fields will be entirely new to you, but there's no need to restrict yourself to areas you've already experienced. Check out Frosh-Friendly Courses to find a good gateway course into an area you're interested in.
Understand that some paths (engineering in particular) will have a series of requirements that need to be addressed in a certain order. Use Explore Degrees and the Engineering Handbook to see what those requirements are, and to incorporate them into your plans.
Recognize that there may be multiple pathways to your specific destination. For example, you can choose many different majors and still go to medical school, and it may be possible to fulfill certain pre-med requirements in a later year, or even after you graduate from Stanford.
Balance your workload. A mix of essays, exams, and projects is generally more sustainable than taking a full slate of classes that all demand the same sort of work.