Whenever we meet with students to talk about their plans, we always bring up two things. First, the first semester in Germany is going to be hard. Although it's relatively easy to gain admission to many study programs, once you're there, you really have to prove yourself. It's not unusual for a third of students to drop a course after the first year.
Second, one of the best parts about living in Germany is that Germans really appreciate the importance of work-life balance. You shouldn't spend all your time in the office, or in the library. If you don't find time for your hobbies, then it's time for a change.
These two factors are very important when you're planning your first semester in Germany. Your first semester is going to be difficult even if you do everything right, but if you take on too much and don't strike the right balance between studying and socializing, then you may not have the outcome you hope for.
When you sign up for classes, you don't have to necessarily take on a full load in your first semester. You'll always be able to catch up, and it's important that you take on a manageable number of courses.
It's also important to talk to your adviser about the courses that you plan to take, and try to focus on taking classes that you're likely to success in. For example, physics students can often choose between the modules "Math for Mathematicians" or "Math for Physicists". Math for Physicists is simpler and requires less work, but fulfills the same requirement.
Also be sure to balance your academic schedule with sports classes and activities on campus. If your life in Germany is entirely spent in the library, your long-term prospects may be bleak. Take advantage of everything German university life has to offer, and your first semester will be full of success.