the story in kh2 doesnt depend on too many things from kh1, but ull be missing a few things that stand out. the game assumes that you know characters, the world stories (each world has a story and some are continued in kh2), and some enemies. i think i favored kh1 overall, but the battle system in kh2 is so ridiculous that it might be hard to go back after kh2.
also u should get COM. it explains the antagonists a bit, a side story (a very important one), and a specific character that would otherwise make u completely confused about her.