Although it is not absolutely essential that you pursue your undergraduate degree in the field of study you plan on teaching at the college level, it does help to have a degree in a related field. Students will sometimes complete their baccalaureate coursework only to find that they wish to move on to another field of study. If you know what field of study you will be pursuing at the graduate level, your coursework should aid you in your graduate studies if possible. For example, someone intending to teach religious studies should either major or minor in the field of religious studies and, if possible, take elective courses in a related field. Taking courses in Greek, Hebrew or history can all prove beneficial to the student of religion.
Like undergraduate studies, coursework at the master's degree level should somehow be related to your intended doctoral program. It need not, however, be in the exact field of research that you intend to pursue. Some students go directly into a Ph.D. program and pick up the master's degree as a coterminus degree offered with the doctorate. For those wanting to pursue a master's degree without entering directly into a Ph.D. program, taking coursework that prepares you for a Ph.D. or other doctorate program can be helpful. Courses taken at this level generally involve a number of research and reading seminars with other master's degree students and Ph.D. candidates.
Students pursuing studies at the doctoral level generally build upon their master's degree studies, unless those studies are in an unrelated field. Ph.D. admissions committees often look for a diverse pool of candidates to admit on a yearly basis and, as a result, will sometimes admit students who have taken coursework in a different field. Students pursuing doctoral studies in a different field should be prepared to show how their bachelor's and master's level coursework may directly relate or influence their doctoral studies. Ph.D. courses are generally similar to those taken by master's degree students and include a number of reading and research seminars. Additional courses may be required in research methods, quantitative analysis or some other type of hands-on work in the field. For instance, Ph.D students in the field of archaeology may go on archaeological digs in order to obtain course credit and gain experience in the field.
The Chronicle of Higher Education is a good place to look for positions as a professor. The job description for each position should give a clear indication of which subjects or areas of expertise are necessary in order to teach at a particular school. Students should have a clear indication of which particular area of expertise they will specialize in and what courses offered at their institution correspond to that field.