Neither of these are very common terms. "Greenhand" or "green hand" can mean an inexperienced sailor especially on a whaling ship, which is a ship that hunts for whales. There is also the term "green thumb" which is said to be had by someone who is good at gardening. ("She grows so many flowers. She must have a green thumb.")

Sezme gave a good answer. I'll just add that "newbie", "novice", or "rookie" might work for 新手, depending on the context. Interestingly, for the opposite (老手), we can indeed say that someone is "an old hand" at something.

I don't know what either of these are.

If your context is with respect to whaling then yes both are correct.