What I did is have my brother prepare a Birth Certificate in English from Municipality from Nepal. I submitted that Birth Certificate and a copy of Character Certificate from SLC that has my date of birth. That was good enough to USCIS.

My GC is already approved. So don't worry of having an Affidavit as mentioned by others that is basically applicable to Indian nationals. My lawyer has also suggested me to have affidavits for my case too. I told them that I was gonna go with what I had. I prevailed.

Good luck!