There is no certificate. "Greenhorn" is simply an English term meaning "a beginner". It has an "old west" connotation to it in the U.S.A., so it was adapted on this site. Anyone who joins is labeled a greenhorn until they make <some number> of posts - I don't recall the exact number.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors