It's a good idea to plan ahead, which means identifying the electives you want and organizing your common core sequence in order to have the appropriate prerequisite at the right time. It also means you should have completed the calculus sequence followed by linear algebra and analysis and optimization.
The primary purpose of the upper-level electives requirement is to solidify your understanding of the common core concepts and tools by applying them to some specific set of problems or issues. A secondary purpose is to provide you with background in a particular area on which you can subsequently build in your senior thesis or seminar.
3. one semester of undergraduate seminar in mathematics (MATH UN3951 or UN3592).