If you look at your spells you will see a number in parenthesis. If it is green, ability power will increase that number. If the number is orange, then attack damage will increase that number. That in a nutshell is how to tell what stat your character will want over the other. Of course, many heroes use a combination and you have to use your own judgment or read forums/strategy guides to see what is usually the most viable option, many characters have many people that will argue one side over the other so there usually are multiple paths for each character. If you are using Rammus, his passive makes his attack damage increase with stacking armor. So if you get items with armor on them, you will also gain an increase to attack damage. If you type "rammus" into the search bar at the top of the forums you should be able to find some strategy guides from players that should help you in the right direction. I hope this helps.