There is the helpfile, which contains almost all of the on EVENTS, commands and identifiers used in mIRC scripting. Type
/help to open it.
There are also many good tutorials available on the internet, try
www.mircscripts.org or searching
Google for tutorials.
For help with a specific mIRC function you can type
/help [color:green]topic[/color] e.g.
/help on TEXT.