Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
A simple explanation...
Alistar is a fairly difficult tank to play and master. I don't pretend to be good with him I am merely showing an example of a build that has proven itself to be amazing time and time again. If you look closely you'll realize it's a hybrid build with a little bit of AP and some health as well as attack speed. The attack speed is for the simple reason that Alistar (being a tank is hard to last hit with.
Mana is also a problem with Alistar as you probably should be spamming his heal. Thus the reason for going after Tears first. You will literally never have mana issues again. Turn it into Manamune after you get some serious attack speed and it buffs your attacks for more Damage Per Second. If you've played alistar before which I hope you have since he's a fun character, you'll notice that he's pretty damn slow. Boots of Swiftness allows for him to compete with even those speedy little buggers like Teemo and Shaco.
The simple Spirits Visage is great for Alistar or any tank with a heal of some kind. The first reason is the fact that it's cheap. Alistar and other tanks don't get money very easily so this item being under 2000 is one of it's redeeming qualities. On top of that, Alistar is a health tank but mage bombs can hurt him very easily. A little bit of mgic resist never hurts. The final Item really can be started work upon at any stage in the game but since it's expensivew and Ali is a tank enough you shouldn't really need it but the AP makes Alistar a god on the Field of Justice.
Very simply put this isn't really a guide but more a recommendation for those who want to try out this difficult but insanely fun tank. Good luck out there and have fun!!