Shortly before your plane lands, you will be given a customs declaration form to complete. Fill in the form accurately and honestly; ask a flight attendant for help if you have any questions about the form. You will also be asked to complete an "I-94" card (Arrival/Departure record) which will become a permanent part of your U.S. immigration record. This card will be stapled into your passport -- DO NOT LOSE IT! After getting off the airplane, proceed to Customs without delay.
At customs, first show your passport, visa, I-20/DS-2019, I-94 and any required certificates of inoculation. Next you may be asked to open your suitcase and other bags. A customs officer will ask a few questions. If s/he is satisfied, s/he will let you go. Remember to speak English clearly and loudly so that the officer can understand and hear you. If the officer seems to have difficulty with your accent, you will usually have more success if you speak slowly.
Once you clear customs, proceed to the terminal for your connecting flight. In some cases, you may need to take a shuttle bus to the appropriate terminal. Be certain that you check your luggage only with an airline clerk wearing a uniform and nametag.