A practice booklet can be found on the ETS webpage here. A description of the Physics exam can be found here.
Although a few students take the Physics GRE in the spring of their junior year, most take it in November in their senior year. Don't wait until the December date, because the scores might not arrive in time. Save the December date for a retake, if you feel that you need to do that; but again, the scores might not arrive in time. Take one of the practice exams early, so you can see what topics are covered and what the problems look like. Save at least one exam for a few days before the actual test, so you can get your mind in test-taking mode.
Don't forget to sign up by the registration deadline.
Here is a GRE Guide from OnlineColleges.net. This is a guide for students planning on taking the GRE. This guide covers all the important subjects and sections of the GRE, how to study and provides additional online resources for students seeking additional help.