Well if you're planning on applying to schools within the US you have to do undergrad first...so pick a major, which major is most likely to get you in to med school is kind of up for debate...I've heard that music majors are most often admitted.

Anyways, regardless of which major you pick there will be certain premed courses that you'll be required to take, such as the slightly harder version of organic chemistry. Since you'll be going to a UC the college counselors will tell you the specific classes which you are required to take to fulfill your major and premed requirements.

Then during your junior year of undergrad, you'll need to take the MCAT exam. It's important to score well on this as you'll be submit this along with your transcript to medical colleges. Many students take prep classes for it and turn into hermits while studying for it.

After that you apply to med schools and keep your fingers crossed.