There is a whole lot of resources that are at disposal online to know how to start with adoption from other countries as a US citizen. Please research further. Even in USCIS portal too, they have resources.
Some thing that might be a little difficult as US citizens:
- you have to stay with the kid back in Nepal for at least 2 years, I do not know how possible it is in your case but, if working remote is possible for you then, maybe you can think about it.
- USCIS will look into it that you have stayed with the kid in the kid's home country for all these 2 years and have taken care of all amenities like the kid's education, healthcare etc. You have to do extensive documentation of all papers.
- Then only you can start the petition with USCIS to adopt the kid. I am sure the local Nepalese ministries "Children welfare" etc. will all get involved to release the kid from Nepal.
- You have to do all these before the kid reaches 18 years old.
- USCIS will let the Children welfare association in US know about your intent and then send some officers to do you home study and interview your family in US as well.
- They will make the decision only after you have fulfilled all the requirements and looked credible people they can let to adopt an orphan.
- The whole process will take time (2 years of co-stay + 2 years of filing) maybe 4 years max, if you are resilient enough, this shall pass.
(P.S. If by any means you can bring the kid to US in some kind of visa, then the process will be easier as then you can stay with the kid here itself for 2 years and then start the petition. But US embassy in Nepal will give visa to some kids with no parents that easy, it is highly improbable.)