I think I might look over my shoulder to check if Karel May was behind me if someone called me a greenhorn.
For beginner: newbie, I suppose, or as mbshu said, noob (gaming/internet context, pejorative - You're such a noob, bunch of noobs) ...
Amateur is often used pejoratively, more slangy and less concrete is "wannabe".
Within specific groups, people who don't fit in or know the ettiquette are sometimes called "tourists" by those who do (I'm thinking of mountain biker friends, who scoffed at me at length for "being such a tourist" on the downhills, but I've heard it in other contexts too) - that is definitely group specific though.
