Ignore the rest of these suggestions. Besides that luadocs link. That one is good.
Learning lua is best through a game because it's a SCRIPTING LANGUAGE.
Learning Lua isn't best through cortex command at the moment. CC's implementation is varied and pretty disgusting. Try learning it through a game like
love or garrysmod or crysis or something(Only until you get the ropes and basic syntax down), then take a look at other people's mods, try things out to learn how they work, and you'll eventually have it down pat.