For that i recommend you to pickup the mIRC Help File avaliable in many sites of mIRC Scripting, and when you got the Help File you need read some topics like On Text Events and some identifiers like $nick $chan, etc.
Read more about as possible you can, to understand how it works and then you be able to do such Bots too =)!
Any problems or questions ask here for help.
Thats all
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