The I-20 is the official document that your university sends
you after you have been admitted there and have completed all the
admission steps.
Once you receive your I-20, you can apply for your visa.
Important:
Only with the printed I-20 form and the visa in your passport can you enter the USA. It is therefore a very important document that you must take good care of. Before each return trip to Germany, have it stamped on page 2 at the International Office.
Here
you can see an example of the 1st page of an I-20 with the data you
will be asked for in the visa application. In the original I-20, your
personal information is entered at the top. You have to sign the I-20 on
the 1st page (do not fill in anything else) and send it back to the
university by email.