The best place to start is by reading a couple of tutes
snYpir's Friendly Intro to Code Snippers
Johan Gustafsson's Scripting Guide.
There are other tutes as well of course, but start with those. Read both the Unofficial command reference guide and also the online version of the official comref from front to back, like a book. Don't try to understand every command, just get a feel for what's there, what's possible.
There is also the Tutorial Mission: play it then take it to bits to see how it works. It has several very simple scripts that provide working examples.
Finally, or perhaps firstly, have a really good look around the Editors Depot. Make sure you know what's there. There are of course many scripts that you can learn from. Some are of better quality than others - don't assume that just because it's there it's a shining example of how to do things.