The department has a list of faculty members who serve as undergraduate 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].
Together with your advisor, you will review the departmental requirements for the major, review your progress toward meeting the College's general education requirements, and work out a plan of studies for the next two years. That two year plan is only provisional and is easily changed - we know that your interests and plans might shift as you get deeper into your program of studies. But it is a reasonable first step in designing your major, tailoring it to fit your interests and long range plans, and finding ways to build intellectual coherence into it.