The department office has a list of faculty members who serve as undergraduate major advisors, and normally students choose one of them. Your initial choice of an advisor is easy to change at a later date.
Once you have decided to major in mathematics, the next steps are to see one of the Mathematics Department's major advisors and complete the Declaration of Major Form [pdf].
You will review the course options, the requirements for the major, your progress toward meeting the College's general education requirements, your interests, and your long-range plans with your advisor. Together, you will work out a two-year plan of studies. This plan is provisional and is easily changed - we know that your interests and plans might shift as you get deeper into your studies.