Earn the underlying degrees. Most PhD programs require candidates to have at least a bachelor's degree in their chosen field, or even a master's degree. You will need this basic education as a foundation on which to build your PhD knowledge.
Find a PhD program. Once you meet the initial requirements for admission, find a PhD program that fits your needs. Make sure the program is well-respected and see what kinds of jobs those that earn PhDs eventually pursue. Apply to at least three PhD programs to increase your odds of admission
Master the subject. Each PhD program has different requirements for gaining the PhD. All will require you to master a subject by class work and intensive research in your chosen field. This research will eventually form the basis of your dissertation.
Write your dissertation. A dissertation is a formal paper written on a subject you choose demonstrating your mastery of that subject. You will have to present your dissertation to faculty members and defend it orally before earning your PhD.