on *:join:#:{ notice $nick MESSAGE HERE }

Replace MESSAGE HERE with your message, you can use colors with the basic color rule and bold and underline. Variables:

$nick = Person who is getting the greet message

$chan = Channel they joined with your bot in it

You may want to include your trigger symbol. (i.e. @ or !) And some other basic commands on your bot. (i.e. Command Menus)