There's not a particular place you can learn to script. However, many say to start at Aliases. Type
/help Aliases in mIRC. Popups are quite easy to learn too. You may wish to see
/help popup menus and
/help Remote Scripts too.
On top of the relatively simple help file (it's a better reference than teacher), you can use many websites which have been started up by users of mIRC whereby people have submitted their own scripting tutorials. Such sites include
www.mirc.net and
www.mircscripts.org - click on the 'Tutorials' section.
..And on top of that, just plain experience helps a heck of a lot
Regards,