<p>Generally you need a German Abitur or an equivalent qualification to be admitted to a German universitiy. In my days American high school diploma did not count, you had to have one year of college or even a AA degree to be admitted.
If you can fulfill these conditions it not difficult to get into a German university since a certain number of places are blocked for Germans and reserved for international students.
Physics, however, is considered one of the hard subjects and the drop-out rate is high.
Good luck with your application.</p>