Write an article based on this "Make the mental switch to research. Take a break if you need to. Do some research into topics you love. Learn about professors in PhD programs. Apply to a PhD program."
When you're an undergrad or even a master's student, your primary focus is on taking classes and learning from your professors. While you'll still likely have some required courses as a PhD candidate, the focus is placed on you developing your own research. You're producing the content instead of merely consuming it. If you're not sure exactly what you want to research as a PhD candidate, it can make sense to take a break after your undergraduate or master's program. Of course, you don't want to wait too long, but you can take a year to figure out the direction you want to go in. Take the time to gain some experience. Apply for lab positions so you can get a feel for what it's like to do research in a lab full time. If you're still in undergrad, try taking classes in topics that interest you or in topics you haven't explored yet. When you come across things that you find interesting, dive deeper. Read more outside of class to see if that could become your PhD focus. Often, your "original" research may just be something you work on under the direction of a professor. That is, you may be assigned your research work and perform the research with guidance from the professor. Choosing a school with professors whose research you enjoy is a great way to focus your work. As your work gets more individual, you want to work with professors who have similar interests. Check out the requirements for each program on the school's website. Every department has different requirements for who they admit to their program. The earlier you understand what they will be looking for in different schools, the higher your chances will be to prepare a great application. Often, you'll need to have a certain level of knowledge in the fields of mechanics, electromagnetism, atomic and nuclear physics, quantum mechanics, and advanced calculus.  Submit all the appropriate paperwork for your application, including your transcripts, academic references, and your basic application.  In many cases, you'll need to write a personal statement or research proposal, as well.