Well I've been doing some research on it and pretty much have it figured out but there are still some things I don't understand like do guns have a range value and how to use the spells. And is there a rule book that I could buy that have them in it?
The Age of Cthulhu line of adventures is designed to be used with the Call of Cthulhu RPG published by Chaosium. All of the adventures require those rules in order to be played, much like Dungeon Crawl Classics adventures require the Dungeons & Dragons rules in order to be played.
Gun ranges and specific rules for spells aren't included in the Age of Cthulhu adventures because it's assumed you have the main rulebook as a reference for those sorts of details. All the answers to your questions can be found in the pages of the main rulebook.
This is the current version of the main rulebook that's put out by Chaosium:http://catalog.chaosium.com/product_inf ... 90063aa077
If you go to places like eBay or Noble Knight Games, you should be able to score a copy of the main rulebook for about $25 or so.
Hope this helps!