First, you must understand that a Ph.D. is a tremendous commitment, taking anywhere from four to 10 years to complete. Determine your motivations for wanting to pursue a doctoral degree, and honestly ask yourself if you have the desire and patience to pursue advanced study for the better part of a decade.
Ensure you have adequate academic preparation to pursue a Ph.D. Most Ph.D. programs require advanced coursework in your subject, as well as a rigorous comprehensive examination before you begin to write your dissertation. If you do not have adequate background knowledge for these courses, consider making up these deficits by taking undergraduate courses to catch up. Maintain high grades in all undergraduate coursework.
Select the program you would like to attend. Things to consider include: whether your studies will be funded, either through scholarships, research assistants or teaching fellowships; program requirements, including courses, examinations and dissertation requirements; academic climate, are other scholars working in your field of interest at the university?; and where graduates of the program work when they have completed the program.
Select a doctoral mentor or supervisor. You should choose someone who works in a field that is of interest to you. Things to consider when choosing a mentor include where their former graduate students are now working -- you may follow suit when you graduate -- the reputation of your mentor among other scholars, and whether your personalities are compatible. This last point is important, because you will be working closely with the advisor until you have completed your dissertation, which may take a number of years.
Choose a research area that is of personal interest to you. Also consider areas that are popular among the academic community, as more funding may be available for this type of research. You may want to work as a research assistant before you enter graduate school to see if you enjoy doing research, because you will be doing this type of work for many years.