An I-20 is a form that U.S. schools issue to international students to indicate they have met all admission requirements to enter that school. If you live outside the U.S., you will need to take the Form I-20 to the nearest U.S. Consulate in your country, along with a valid passport, TOEFL score, financial support documents and school transcripts, to obtain a F-1 Student visa. It is important to remember to sign your I-20 prior to entering the United States.
If you are already in the U.S., the form may be issued only after the release date given to you by your previous school. That is usually the last date that you have attended classes.
To allow time to overcome any unforeseen problems that might arise, students are encouraged to apply for their visas several weeks before they plan to travel. Students may not be issued a visa more than 120 days before the registration date noted on the I-20.