Its better to get the birth certificate. You can ask your folks/relatives to get one made for you from your village. If you cant get one, you can also have affidavit of birth certificate from someone who witnessed your birth. There is no particular format to it but he/she can write a letter to USCIS indicating that he/she knows when you were born based on personal relationships and that there was no custom to register births at that time in your village. Please make sure to have that letter notarized from the public notary. This should suffice if you dont have birth certificate.