Do you plan to study in Germany? If you plan to enroll in a German-taught program, you need to first prove your language proficiency.
There are several ways that you can prove you language proficiency, including using your experience studying in the United States. But most programs will require that you show C1-level proficiency by passing one of several government-recognized tests.
One of these is the Deutsche Sprachprüfung für den Hochschulzugang (DSH). This exam is offered at universities across Germany and consists of listening, reading, writing, grammar and speaking sections.
Before you take the test, you can prepare by using practice exams offered by universities across Germany.