First, EliteKill has written a nice "Modding for Dummies". Even if you have not done anything like that before, you should be able make some fun modifications to your game. This guide is likely to cover even more things in the future.
Second, myself finished the first version of "The basics of Heroes V scripting" (a pdf compressed to zip). It is a supplementary matterial for the official editor documents and describes the scripting language. Robenhagen even gave it a professional look!