Charles’ recommendation is the single best way to learn. Set a particular goal to achieve through scripting, and before you go to the Lexicon, reason through in your head based on whatever you know about NWScript about how you think you’d go about accomplishing it. Check to see if the functions you think you’ll need are already written (often they are!) and assemble your script. You’ll be surprised how much else you pick up on the way, and by the time you’ve figured out how maybe 10% of the included BioWare scripts work, you’ll be able to guess how the other 90% are written based on their names alone.
You’ll screw up a lot, but we all did. In the end, the small goals you set for yourself will serve as your ‘homework assignments’ and they’ll get progressively more complex as you become more ambitious.
For my own part, I was fairly proficient scripter back in the early 2000s when the games were first released, but after finishing university and starting work in a field completely unrelated to computer programming, my skills fell to rust. With the release of the Diamond Edition on GOG, I challenged myself to build a PW for my friends, and to rebuild my skills, I started a small module with a couple of areas (several interior levels of a large home, the grounds outside, a subterranean level, etc) and in each room I set up different challenges for myself; for example, creating a ladder/trapdoor between one level of the house and the roof, creating two possible, morally-opposed henchmen that would react differently based on which was recruited first, randomizing the placement of a secret note in the inventory of various bookshelves, and so on. The little test module itself became so interesting in the end that I’m likely to release it in a few months (well, whenever it’s done) as a standalone adventure.
I’d recommend doing the exact same thing. Start a little module with no purpose in mind other than to be a series of disjointed experiments, and eventually the workspace will inspire you further until you’re doing things you’d never even thought you could.