Lots of opportunities are available to those with a bachelor's degree in mathematics. In some fields, such as biostatistics, finaincial mathematics, or operations research, a professional master's degree is preferred (or at least qualifies one for a higher salary). In research mathematics, a Ph.D. is required. Keep in mind: graduate school in the mathematical sciences is often free. Most Ph.D. programs in pure mathematics have financial support available in the form of tuition waivers plus a research stipend or a part-time teaching/grading job. This is also true for Ph.D. programs in statistics, applied mathematics, computer science, and operations research. Financial support for master's degrees varies wildly from field to field and sometimes from school to school; it's rarely available for pure mathematics, but is much more available for applied mathematics, statistics, financial mathematics, and biostatistics.
How should you go about applying to doctoral programs? Lynne Butler at Haverford provides advice and a timeline to which the following might be added:
The Registry of Undergraduate Researchers
might match you with a graduate school.
Maybe gradschoolswantyou.com will provide you with a recruitment opportunity.
Here are listings of professional master's degree programs. There are roughly 100 of them.
Want advanced work in actuarial science? Check this list of programs.
Make your own rankings for Ph.D. programs (that is, rankings based on criteria you choose). This site also has the most recent ranking of programs by the National Research Council as of 2010.
US News also ranks graduate and professional programs; check to see if your library has online access or the print edition. Also look for The Gourman Report, most recently published in 1997 but still reasonably relevant. Harvey Mudd maintains a list of graduate mathematics programs ordered by the National Research Council's 1995 rankings.
Here are hyperlinked lists of U.S. doctoral programs in mathematics and applied mathematics/operations research and statistics/biostatistics and mathematics education, so you can check out individual departments.