Places I used when I was starting are:

Wikichip as noted in an earlier posting - I use this as a reference rather than a 'how to'
http://www.virtual.net.au/~bekar/mirc/Dialogs-HOWTO.txt.html
http://mircscripting.info/forum/viewtopic.php?t=414&sid=f691779ce89dec72d4554c297f625e45

If you want to see a simple dialog in code and can try it out yourself have a look at

https://forums.mirc.com/ubbthreads.php/ubb/showflat/Number/263983/Searchpage/3/Main/50614/Words/%2Bsearch/Search/true/re-findtext-v-ctrl-f-pop-up#Post263983

where I posted a simple dialog that mimics the search for text in any window (basically a duplication of Ctrl-F inside mIRC)