To apply for a PhD program, one must first complete a Bachelor and a Master degree in a particular field. The applicant may hold Bachelor degree in the same or a related field, before applying for a Master, and likewise for Master before applying for a PhD. For example, social anthropology may be a PhD for Master in Sociology and Master in Social Anthropology alike. Master in Criminal Justice may apply for PhD in Forensic Science as well as a PhD in Criminal Justice. Each PhD program has its particular prerequisites, so consult the institution for detailed requirements.
Research is the key to PhD application. PhD applicants already have vast knowledge of the topic they wish to specialize. The research process includes thorough study of expert literature as well as practical research work. You may wish to become a research assistant to improve your knowledge. That means you will have a supervisor and a mentor. Connecting to other researchers interested in your field of study is also a good option. Through experience exchange, both sides are likely to profit in future studies. That will also give you an idea if the desired PhD is exactly what you want.
Graduate Record Examination (GRE) is prerequisite for most graduate schools. Educational Testing Service (ETS) administers the tests, which cover many subjects. Choose a topic depending on the desired PhD program. General exams test verbal reasoning, quantitative reasoning, critical thinking and analytical writing and are not connected to any specific field. However, GRE subject tests offer a large variety of specific fields: biology, biochemistry, cell & molecular biology, computer science, psychology, literature in English, mathematics, and physics. If business studies are your choice, take GMAT (Graduate Management Admission Test). GMAT tests mathematics and English language. Many business schools accept both tests, but GMAT is preferred.
When applying to a PhD program, you need submit your Curriculum Vitae (CV). A good CV should include your educational qualification, research experience, GRE or GMAT certification, your interests areas, all experience in the field (both academic and non-academic), all other relevant skills (for example, foreign languages and computer knowledge), your awards, publications (if any), and other interests. Include names and contact information of people who may act as your referees (your mentors and fellow researchers, and former employers). Letters of recommendation of all the referees should accompany your CV and PhD application.