The short answer is you come to see me and fill out a major card. Of course, you need to do a bit of homework first so that we can select the right courses for you -- or at least a first approximation. (Filling the card out the first time can be a bit daunting, and many students revise their plans as they learn more about mathematics.)

The requirements are pretty straightforward. You have take all the prerequisites: Math 3, 8, 13 and 22/24. Of course, you can use advanced placement credit or placement to deal with these. Then you must take 8 more courses which must include an algebra course (Math 31 or 71), an analysis course (Math 35, 43 or 63) and one of your eight courses must satisfy the culminating experience requirement (these courses are listed in the ORC which is a good thing to be looking at right now).

I strongly encourage anyone who wants to major in math to take either 22 or 24 as soon as possible. After that, you'll have a better idea of how to craft your major.