Good thing you are thinking about your future. Most applicants to business school are looking to make a career change and come from all sorts of unique backgrounds. A few calculus courses and maybe some basic economics would be beneficial, but certainly not necessary. These will look good especially if you are studying something “non-traditional” such as English or history. But don’t worry, plenty of kids from those majors get into the top schools every year.
Yes, schoolwork is important in the application process, and you should try your best to maintain a GPA in line with the schools you hope to get into. However, don’t get all too caught up in grades. Make sure you take on an active participation in ECs at school. By active, I mean more than just a plain old member. Seek leadership roles. Make a difference in your community. These things will not only prepare yourself as a leader, but will also make a big difference in your application to graduate school.
The title of your degree won’t make a difference if you don’t have concrete goals and a story that is supported by your past actions. Why do you need/want an MBA? What do you want to do in 5 years? 10? 30? While in undergrad you don’t necessarily need to know the answers to these questions, you certainly need to figure them out once you get some meaningful real-world experience.
One piece of advice…if you are serious about an MBA, take the GMAT as early as possible. Score is good for five years.