Extending SharePoint Calendar webpart

I am new to SharePoint development so forgive me if this is the wrong forum.  I don't know where else to go and there are so many brilliant people on here.
As your reading be aware that subscribing to Alerts by email/sms and RSS feeds will not solve my problem.
How can I extended/edit/modify a list in SharePoint so that my users can sign up to be notified about every change in the list, then once something on the list has changed, some custom code is executed????
 
Thanks
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