Go to your ward office with your Citizenship and have them made one. They will probably know what to do but it should include the following:
Your Full Name
Date of birth
Place of birth
Names of both parents

Also see the following links:
http://nepal.usembassy.gov/bc.html
https://travel.state.gov/content/visas/en/fees/reciprocity-by-country/NP.html

Good luck.