I learned athena scripting years ago, when I was just a regular RO player on one of private servers. I was learning the game, and at that time, I found the easiest way to learn about RO quests, spawns etc was to look up eAthena npc folder. But I had to understand it first. C-like syntax was easy, cause I had already known C. I've read every letter in doc/script_commands.txt to understand variables and other stuff that was athena specific. After that and learning from few scripts that I was interested in as a player, I find out that I'm able to more-less understand any script. I learned good patterns and practices by reading more and more scripts that were somehow relevant to me, then I applied as Scripter to the server I had been playing.
Even with syntax highlighting and some basic autocompletion, I still need to refer to docs. There are just too many script commands to remember them all.
TL;DR> read docs, read scripts, practice