One thing you need to know when writing your own scripts is how to look at a file in a hex editor and figure out how it's put together. This is necessary because otherwise, you don't know what script to write! The only real way to do this is with experience, there's no simple way to just know
it, I've learned it over time by simpling trying myself, seeing other people figure it out and just expanding my experience like that. Some files are still too hard for me, but when someone else figures it out I look at what they did and try to remember that for next time.
As for the the actual language, it's fairly straight forward, especially if you've done some programming before. You should be aware that BMS isn't a proper programming language, it's a scripting language that, while it uses a lot of programming concepts, is also different from most languages. For that reason I personally wouldn't recommend it for a language to begin learning programming in. That said, it is perfectly possible to learn it relatively easily with no prior programming experience.
Luigi made a pretty good tutorial here
, it's not super-simple but with luck you should be able to follow it. Those tutorials on Xentax are also good starters, to make you a bit accustomed to looking at files as well as just the syntax of the script.
And keep in mind that you need to always look at quickbms.txt. Nobody ever remembers all the commands, so like any language just look at the reference if you don't remember a command properly, or even to find new ones.