Step 4: Begin to complete requirements (coursework, exams) and look for a research group, topic and advisor.
Complete general requirements first, unless courses may help you choose your research topic.
Your advisor matters even more than the topic. Choose carefully. An amazing research subject and project can be ruined by a terrible advisor. The support of your advisor will be huge deciding factor on your Ph.D. and future career. Ideally, choose someone who truly wants to and knows how to support Ph.D. students through a successful Ph.D.