There are many things to check before picking a discipline. It wouldn't hurt to make some preliminary research, and wikipedia can be a good source for that. If you are considering mathematics, why not go to http://en.wikipedia.org/wiki/Mathematics? Hardly a bad place to start.
Another thing you might want to check is the average and/or median salary for that discipline. You can find that out in your national bureau of labor statistics. This is the U.S. bureau of Labor Statistics' page with data about mathematicians: http://www.bls.gov/oes/2009/may/oes152021.htm.
If you've decided mathematics is your desired field of interest, one of the next steps would be to find out the right university: Find the Top Universities for Mathematics:
The following ranking is the ARWU ("Academic Ranking of World Universities") ranking for Universities, solely for the discipline of mathematics:
| World Rank | Institution* | Country | Score on Alumni | Score on Award | Score on HiCi | Score on PUB | Score on TOP | Total Score |
|---|---|---|---|---|---|---|---|---|
| 1 | Princeton University |  | 85.8 | 100.0 | 77.0 | 70.9 | 92.9 | 100.0 |
| 2 | University of California, Berkeley | | 56.2 | 53.5 | 95.2 | 78.7 | 86.0 | 93.8 |
| 3 | Harvard University | | 85.8 | 70.7 | 83.5 | 62.0 | 91.4 | 93.6 |
| 4 | Stanford University | | 0.0 | 26.7 | 100.0 | 69.3 | 99.6 | 85.0 |
| 5 | University of Cambridge |  | 88.9 | 80.2 | 59.1 | 57.3 | 84.7 | 84.9 |
| 6 | Pierre and Marie Curie University - Paris 6 |  | 64.9 | 0.0 | 69.9 | 89.7 | 74.3 | 77.5 |
| 7 | University of Oxford | | 45.9 | 53.5 | 64.7 | 64.3 | 80.1 | 77.4 |
| 8 | Massachusetts Institute of Technology (MIT) | | 0.0 | 37.8 | 64.7 | 73.1 | 87.7 | 74.0 |
| 9 | University of Paris Sud (Paris 11) | | 51.3 | 80.2 | 26.4 | 73.8 | 77.2 | 73.4 |
| 10 | University of California, Los Angeles | | 0.0 | 59.8 | 52.8 | 69.5 | 83.3 | 72.0 |
| 11 | University of Chicago | | 32.4 | 26.7 | 52.8 | 60.6 | 96.3 | 71.2 |
| 12 | University of Minnesota, Twin Cities | | 0.0 | 0.0 | 81.4 | 65.6 | 91.2 | 71.1 |
For more, you can go to http://www.arwu.org/SubjectMathematics2010.jsp.
