First step to applying to graduate school in USA is taking required exams. These exams are GRE Subject (Graduate Record Examination) and you will take different GRE depending in what field you are applying to.  If you want, for example to, apply for chemistry you will have to take GRE Chemistry, or if you are applying for psychology you have to  take GRE Psychology. I recently had experience with GRE physics and i'll share my thoughts and what I acknowledged during that process. Hopefully someone might find this interesting and valuable.

Gre Physics is weird test, it's like nothing you have ever stumbled upon before. 100 problems, some with simple calculations, some just in form of a question, in 170 minutes. You have to be fast , you have to be prepared, you have to be good in dealing with pressure, you have to be concentrated. So let's start.

Introduce yourself with test

Test is structured in that way that it covers every topic in undergraduate physics courses.

Classical mechanics 20%

This is the cornerstone of the GRE test. Of course, this subject is the first thing you learned in your freshman year, and many of these topics are actually covered even in high school. In my opinion this was the easiest part of GRE, I was more than happy every time i had seen classical mechanics related question.

This part includes problems and questions involving classical high school problems with blocks on ramps, or hanging block, friction, kinematics, energy conservation laws and related problems, Lagrangian and Hamiltonian mechanics, springs, fluid mechanics and similar. You should not have any difficulties with any of these topics, and if you have, then maybe you should reconsider your decision to apply for graduate school.

Electricity and Magnetism 18%

This is also one important part of your undergraduate education, and important part of GRE test. I had certain problems with this part, mostly because of having not so good lecturers in my undergraduate classes but generally speaking this part is maybe just a little bit harder than classical mechanics. This part includes Electrostatics, Magnetostatics, Electrodynamics and related problems and questions. You should be familiarized with Maxwell's equations, Ampere's law and similar.

Optics and Waves 9%

This part is interesting because in my opinion it contains mixture of very basic and some advanced problems. Here is important to to remember wave equation, refraction formulas, some usual polarization formulas interference and diffraction problems, such as double slit and single slit interference, reflection and refraction (here is important just to know how to deal with lensmaker formla).

Thermodynamics and Statistical Mechanics 10%

Slowly we are getting to some more advanced topics. You should not have problems with thermodynamics, because problems involving thermodynamics are basic, but statistical mechanics problems might be tricky. That was my impression, but as far as I know other people also had difficulties with this topic.

Quantum Mechanics and Atomic Physics 22%

The biggest part of exam is this. And the hardest. Quantum physics certainly seemed bizarre when you first introduced to it. And if after graduating, you are still noy familiare with quantum physics, don't worry you are not the only one. Well, it's hard to sum the most important things in this topic, especially myself not being an expert in quantum physics, having just one undergraduate course related to it. But give your best and hope for the best.

Special Relativity 6%

Although you might get scared by relativity, this topic includes only basic thing such as time dilation, length contraction, relativistic Doppler shift. Just be focused during calculations, and you should not have any problems with this.

Laboratory Methods 6%

Most annoying part of the test. Myself being an astrophysics major undergraduate, had almost no laboratory experience at all and this topic required to be familiar with statistics, error analysis, electronics, laser and interferometers, just not worth the time.

Specialized topics 9%

What makes GRE test unique is having a section that is completely randomized and will test your physics general knowledge. Most of this section will be questions on nuclear physics, such as quark structure of certain particles, and astrophysics (I was lucky) questions related primordial nucleosynthesis or redshift, and condensed mater physics(i really hate this one).

Source- http://www.mastersdegree.com/exam-prep/gre/subject-specific-tests/

NOTE- Just to know, questions at the test are not sorted in this way, your first question might be classical mechanics related, but second could be quantum physics, third statistical mechanics, and so one. In this way test requires you to be really be extremely prepared and ready to change the topic in just seconds.

Register on time

Test if offered only three times per year, so don't wait for the last moment, register at least two months before the test date.

Preparation

Instead of dealing with numerous undergraduate books, reading thousands of pages, trying to memorize stuff you won't need for the test, my advice is to find some books that are written specially for GRE preparation.

My warmest recommendations go to the book ''Conquering GRE Physics'' by MIT grad students, who were motivated to write this book by the lack of appropriate preparational material when they were grad school applicants.

Book is full of useful advice, tips how to think under time pressure, different shortcuts that can be useful when you are running out of time, such as to use  dimensional analysis (GRE actually loves to do this, to give some extremely hard problem, but among possible answers only one is in appropriate dimensions), limiting cases and similar.

Tips

Start early with your preparation process, 6-10 months in advance is enough. Don't leave everything for  the last moment.
Before exam sleep well, eat well. Don't panic.
Practice at home under time limits. Huge threat is to be slow. Don't forget you have only 1.7 minutes per problem.

GOOD LUCK!