Welcome to the madness!
Rules: Epic Armageddon is the more popular of the two (tbh I've never seen the alternative played). The rules also contain all the 'codicies' and list variants, along with all the FAQs being included on the page where they are relevant. Onyx is the main Epic man and he has a page of useful links set up HERE
. Well worth a look
Also, if you check out some battle reports which feature Onyx, chances are he'll be rocking his CSM. They. Are. Brutal. And should give a fair idea as to where you might start.