You must be fluent in German at a high academic-technical level of proficiency. University entrance exams, reputed to be among the world's most difficult, are in German, of course. So is the visa interview.
Note Germany abolished tuition-free studies for foreign students after a brief experiment with it a few years ago. You will have a lot of expenses, and no financial aid. If you are US citizen, you would have the possibility of financial aid, plus you can WORK, as 80% of American college students do.