There are two sources of information for scripts that come with Ace (well, three): First, is learning the Ruby language. You can't use all of it, but RGSS3 is a subset of it, so you have to learn the syntax and best practices and OO design and all that if you want to be able to write solid code.
Second, is the script library that comes up when you press F11. All the scripts listed in the left pane are fully spelled out, and commented pretty well. I printed them all out and keep them in a binder where I highlighted different things (method names in one color, classes in another, etc...) so I can read them when I'm away from my computer. I also find it easier to write notes and ideas on another piece of paper while I'm reading from the binder - but I'm old school.
Third is the help file: Go to the Contents tab: "Reference Material", the last 'choice' under RPG Maker VX Ace gives a lot of good definitions - particularly regarding parameters. The rest of the stuff you need is listed under RGSS Manual. The RGSS section lists the classes and methods specific to RPG Maker (a separate list from what you see in the script editor), and anytime you see (RGSS) after something, it means that it either is something RGSS has added or changed the behavior of from 'standard' Ruby. For those you don't get the full code listing, but you get the parameter lists and outputs, which in most cases is good enough. If you want to rewrite those it gets a bit trickier (may want to alias), but should still be doable.
Oh yeah, and you can get help from places like these forums and other community websites/YouTube channels/tutorials...
