In my case, I lived in the USA for a long time now but it's my first time asking for a social security number.
for your case, you'll need to prove that you have never been assigned a social security number. You can do this by providing records about the schools you attended, tax records or something similar.
about the original documents, please give me some examples of the documents needed.
One of the most important original documents required is the U.S. passport.