When a student applies to Johns Hopkins University as an undergraduate they are applying to the whole University and will have access to take classes in the Krieger School of Arts & Sciences and the Whiting School of Engineering throughout their four years. A student is pretty much cross-registered in both schools, and when registering for classes there will not be restrictions across the schools no matter what you have chosen as your academic major. Almost all students split their coursework between the two schools, with many choosing to major and minor across the schools.
During the Admissions process I student will need to select on their application their intended major (actually we ask for two areas of academic interest) but this is for the application review process. A student is not admitted to a specific major but the University as a whole, which means one can change their intended major at any time. Engineerings have until the end of their first year to officially declare their major, while Arts & Sciences students have until the end of their second year.
The one exception to all of this is the Biomedical Engineering (BME) program. Students wishing to enroll in the BME major must indicate BME as their first-choice major on their application. Students are admitted specifically into the BME major, based on evaluation of credentials and space available. Students can be admitted to the university without acceptance to the BME major. Only students admitted directly into the BME program may register for that major, but BME students will take courses in the Krieger School as well.
It is important as you prepare to apply to Johns Hopkins that you make the decision whether BME is really something you are interested in studying. BME is an ENGINEERING major and is for those students who are specificially interested in the applied life sciences. A student more interested in Biology, Neurscience, and specifically the natural sciences and pre-med may find the BME program is not for them.
Finally, in general it is recommended that students take the strongest academic classes they can throughout high school and still maintain the best academic performance possible. We do not have specific requirements for students as it pertains to what courses will be considered competitive but it is important to note that there are differences in the review of Engineers versus Arts & Sciences applicants. For an applicant to BME more emphasis would be placed on AP Physics, AP Calculus, and then AP Biology. For an applicant to a Natural Science major, AP Biology, AP Chemistry, and AP Calculus would be recommended priorities. In the end though, these are recommendations only. The Admissions Committee are the evaluators of the decisions students make throughout high school, and the courses you select will say a lot about what kind of student you will be in college.