Well you can first try to see if your parents or other family members might have a copy of it.  If you were naturalized then you would have had to submit a copy of your birth certificate in order to get a visa and/or green card so it is/was in someone's possession at some time previously.  if you are not able to find it this way, you can try to obtain it from your country of birth.  you don't mention which country, but assuming it is still a functioning country, these records are most likely kept.  you can also contact your country's embassy, if one exists in the US, to try to get some assistance.  if all of that fails, you can get an affidavit of birth from people who were either present at the birth (parents) or family members who were aware of your birth at the time.  please feel free to contact me for further assistance.