If you know for sure that speech-language pathology is for you, your best bet is to major in communication sciences and disorders as an undergraduate. Master's programs will take students with other undergraduate backgrounds, but most programs have extensive prerequisite course requirements that are easiest to cover in a dedicated communication sciences program. At the University of Texas at Austin, for example, students from non-communication science backgrounds need to register as non-degree students and fulfill prerequisite course requirements in psychology, statistics, physiology and other areas before they are eligible for graduate program admissions. Research the master's degree programs that interest you, determine their specific requirements and consider whether you can get the courses you need through your desired undergraduate program.
Although a master's degree isn't a universal requirement to become a speech-language pathologist in the United States, it is the minimum educational requirement in most areas. According to the United States Bureau of Labor Statistics, 47 states regulated speech-language pathologists in 2009, and most of those states required that licensed practitioners hold a master's degree. Medicaid, Medicare and other private insurers require that speech-language pathologists be licensed before they will reimburse patients in their care. For state-specific requirements, the Bureau of Labor Statistics suggests you contact your state's regulatory board. Keep in mind that earning a master's degree will increase your career opportunities, even if your home state does not require it.
Speech-language pathology will stimulate people excited by the prospect of becoming life-long learners. According to the Bureau of Labor Statistics, many states require that speech-language pathologists pursue continuing education credits to renew their licenses to practice. Expect to take a few college courses each year to keep your mind sharp and your skills current. For example, you might take a course that discusses the specific communication issues found in people with autism spectrum disorders, or a class addressing the affects of dementia on communication, depending on your professional specialization.