An "on join" notice? Well, it could be annoying for some people, specially the channel ops, it could be considered as spam if you include a web link.
Here's the code you have to put in the "remote" section (alt + R)
on *:JOIN:#:/notice $chan Welcome to $nick !
This notice will be sent to the whole channel, it's quite annoying if a lot of people is joining a channel.
on *:JOIN:#:/notice $nick Hi $nick and welcome to $chan !
This one will be sent to the joining user only.
by the way, everything you need is in the mIRC's help file, just type: /help on join