This is not a comprehensive guide. This is a build made for feed-back. It will, however contain a small guide to how to use this build. Please do not down vote if you havent tried it. This is my first build and I hope you guys like it.

The runes I chose are what I feel are the best for the situation. If something is wrong with the runes I chose please comment and I'll gladly fix them if I see that you are right. Alistar needs mana early game or else will be mana drained in a small amount of time.
Because you are tanking/ offtanking you need to have more magic resistance in late game or else you'll be shutdown without a problem.
I pick magic penetration for his "Q" and his "W" and also his proc hit with lich bane.

For masteries I went 9/21/0. You don't need 21 points in the offense tree because he deals all his damage through his skills and proc hitting so there is no need for additional damage, whereas he does need more defense to finish his combo and to make sure that the enemy is dead.

Alistar NEEDS speed 3 boots or increased movement speed from an item. Luckily lich bane has increased movement speed XD so you don't need speed 3 boots, however it is still highly recommended.

Lich Bane is the perfect item for AP Alistar - ap, mr, speed, and passive!

rylai's - gives more health which he needs, ap, and slows enemies down from skills.

RoA - give ap, health, AND MANA!

Deathcap - OMG so much ap :D

zhonya's - zomg more ap and armor, and passive. . . yes please!

the order is based off preference, I prefer boots early because of how slow alistar is, I prefer RoA early to make sure I get it maxed, and lich bane fairly early for proc.
Rylai's isn't as important and can wait and robandon is just a cherry on top, while zhonya's is the whip cream under it :p yum.

Once again based off preference. Heals are great for laning early game and awesome save from someone tower diving you. rofl when they die trying to kill you.

Headbutt is your main defensive/ offensive skill as alistar, it depends on how you use it. if your hugging your tower like a boss with low health and they decide to tower dive.
Ult>e>q>w behind ur tower so u can get them trapped under tower fire.

Pulverize is used for its stun early game, by late game this will be one of 3 main weapons. :D
This means that there is no point in leveling it up early.

I chose ghost and exhaust.

Ghost is a MUST you aren't fast enough to run away or chase as alistar so use ghost.

I would normally choose flash and exhaust but because it is going to be gotten rid of, I figured I might as well get used to not using it.

Ignite would be a viable choice instead of exhaust, but when my CCs are on CD I like to have a soft crowd control to chase.

When you have the full build you will want to deal the maximum damage possible to the enemy team.

In a team fight:
Q > Proc > W > Proc > R > Proc > E > Proc

this will deal approximately 600 damage to all enemy teammates hit by the "Q" and stun ALL of them. Then you choose your target or choose before hand, doesn't matter, and proc. Then send them flying TOWARD your teammates to let them know " that guy is dead." Now either proc him again or start to target another baddie.

By the way your heal is pretty insane so if your team or even just one member needs health use it. It's pretty cheap late game and the health is like 500 or something to you and like 250 to everyone else.

Hope you enjoy this build. If you don't like and decide to down vote then please comment and tell me why. Also I am willing to accept constructive feed back and hope to work on this build some I am fairly nooby so any advice is warmly accepted :p