Although there are various other sites around, personally I quite like
this page from the mIRC website, where it is written:
The best way to learn and understand the way mIRC's programmable sections work is by studying this chapter in consecutive order. Really!
The mIRC FAQ is downloadable as a separate item from this website, and while not perfect the mIRC help file can be pretty helpful, too!
Unfortunately, some scripting features are only described in the complete mIRC
versions.txt, which you will need to download from this site as well (only a shorter version ships with the mIRC installation package).
I would suggest starting with simple aliases, moving on to think about popups, and then having a look at remotes. Remember that for 99.99% of us there is ALWAYS something new to learn, and don't be afraid to ask for help if there's something you don't understand - either here or in IRCnet channels devoted to script help.
Good luck, and have fun!
PM