Math_QED said:
You are still in high school and already thinking about a PHD. I think you should first focus on enjoying physics in college and getting good grades.
I can think of several benefits from considering a PhD by the time one graduates from high school or very early in college:
1. The choice of undergraduate institution can have a significant impact on prospects when one applies to physics PhD programs. If one is limited by resources to public schools in one's home state, there are usually one or two choices that are better than all the others (Texas A&M and UT in Texas, LSU in Louisiana, Ga Tech and UGA in GA, UNC and NC State in NC, etc.) Attending the lowest tier BS programs in physics will not serve one as well as attending one of the better public programs in a state.
2. Getting involved in research early can significantly strengthen one's application to graduate school. Skills gained in high school can create much stronger research group candidates for early undergraduates. Many schools delay undergrad research opportunities until senior research projects for students who are not intentional about finding them. In the final year of a BS program, one simply has far less opportunity to complete research accomplishments or impress recommendation writers by the time grad school applications are due.
3. Knowing one is likely applying to graduate school, one can take steps to front load the courses most important to the PGRE so they are completed before taking the PGRE. With all the high school dual enrollment programs around these days, one might even complete the intro physics sequence before graduating from high school. The way prerequisites stack up in most BS programs, this allows completing the most essential courses before one takes the PGRE. An early start in physics coursework also leaves one better prepared for research opportunities.
4. The job prospects with and without an advanced degree are a worthwhile consideration for many undergraduate degrees. Physics is not as extreme an example as pre-law or pre-med, but consideration of employability without an advanced degree is more important in physics than in engineering, education, or nursing. I see lots of physics department web sites touting all the job prospects for physics majors, and many students come away with the mistaken impression that finding a job will be a snap with a mediocre GPA from these mediocre departments. A more sober appraisal of job prospects would be helpful BEFORE students start spending time, effort, and money in these programs.