I feel like half the people I meet at Cal are MCB majors. Not to dump on MCB, it's a wonderful major, but most people choose it as a default. If you are one of those "defaulters", not only in MCB or any other major, seriously reconsider why you're studying what you are. Is it because you want to make your parents happy? Is it because all your friends are studying it? Is it because you really don't know what you want to do?

If you're undeclared, think about what you honestly like. If you're not a science/math person, don't be science math major! Your major should be something that to some degree, is enjoyable. Also, think about what are you good at. Usually, these two go together and can help you narrow your focus way down. If you have an idea of what kind of career you want to pursue, it could help to pick a concentration that helps you reach that goal.
Take some courses from different departments. Fulfilling your breadths and AC requirements can help you do this.
Talk to advisors and other people who are in the majors that you're interested in. They can help you get a realistic picture of what the major is like and the kind of courses it entails.

And if all of this fails and you still end up not knowing what you want to study or find yourself constantly switching majors, take heart. Your major doesn't determine the the kind of career you're going to have or who you are. At the very least, college should be a place where you can explore many different things and have the freedom to do what makes you happy.
If you have questions or are super stressed about these issues, come talk to us in office hours!