Regarding majors, you can major in anything you want as long as you complete the pre-requisite courses for medical school. These include one year General Biology, one year General Chemistry, one year Organic Chemistry, and in many cases Biochemistry as well. Different medical schools may have additional pre-reqs for their applicants. These might include Statistics, Calculus, English Composition, Humanities courses, etc. The key is to major in what you are passionate about. If you are passionate about science, major in a science. If you are passionate about English, major in English. If you aren't sure what you are passionate about, don't feel obligated to pick a major right away. If this is the case with you, enroll in some pre-reqs and gen eds, and you will find what you like.
Go to whatever undergraduate college you think you can be successful at. The prestige at your undergraduate university makes only marginal difference in medical school admissions, if any at all. (Do some searching on this site for more information on the topic. It has been discussed extensively.)
For traditional students(folks going to med school straight from college), one's state of residency is the state he or she graduated high school in. Because you are growing up in California and will graduate high school in California, your state of residency for medical school admissions purposes is California.
As far as retaking the ACT, I feel I should make it known that when applying to medical school, how you did in high school makes no difference. Your retaking the ACT for a better score will only make a difference in your acceptance to undergrad and the financial aid you receive at your undergrad. Your ACT score has no bearing in your application to medical school. I know you know this, I just needed to put my public service announcement out there.
Good luck to you, OP.