Alistar is a great supporting tank. In the next steps i show you how to use his abilitys in good combination, to be a feared warrior. Alistar needs some sense to play, so gain your own experience with him.

I take ,

• 9x( Greater Mark of Magic Penetration)
• 9x( Greater Seal of Vitality)
• 9x( Greater Glyph of Scaling Magic Resist)
• 3x( Greater Quintessence of Movement Speed)

Magic penetration makes it easier to kill minions and farming gold.

Live per level, to get a high amount of live in late game.

Magic resitance per level, because armor will always be more represented as magic resistance.

Movespeed Quintessences are always usefull as tank, to be better in initiating fights. And ganking from behind.

I start with Rejuvenation Bead, Faerie Charm and an Mana Potion. Those items give me the option to stay extreme long time on a lane without getting back until i reach level 8 or 9.
When i get back i buy for Philosopher's Stone and Boots. With Philosopher's Stone you won't get into trouble with your mana consume and the extra gold is a good boost for the next items.
Next i head for Kindlegem to get the CD's of Pulverize and Headbutt low. Then i finish Shurelya's Battlesong. It's a perfect item to support your team and deadly in combination with Randuin's Omen.
Next item should be Mobility Boots. It gives you the option to disapear quickly and after using Cleanse your guaranteed to get out of trouble. Perfect to get a fast gank done very easily.
To get more Health and Mana you should buy Banshee's Veil. Start with a Catalyst the Protector to gain your needed life.
Now you got a high amount of Life and Mana and should buy as fast as you can for some armor, so buy Warden's Mail to avoid damage of AD Champions. Use the Rejuvenation Bead to finish Warden's Mail. The passive of Warden's Mail is really useful and during you buy for armor your Greater Glyph of Scaling Magic Resist should give you enough magic resist to be up with armor.
Now finish Banshee's Veil to raise your health and mana. The passive of Banshee's Veil is one of the best passives in the game.
Now you got a great defense and should increase your life again. Buy Heart of Gold to gain more gold again and finish Randuin's Omen.
Get Shurelya's Battlesong to slot 1 and Randuin's Omen to slot 2 to get a fast usage of their actives.
Your Health should be raised by ca. 1000 health points, so head for magic resist and armor.
That’s why your next item should be Guardian Angel. Buy situational first the magic resist part or armor.
When you finished Guardian Angel nobody will focus you at all because with you Unbreakable Will, Guardian Angel, Shurelya's Battlesong, Randuin's Omen and Cleanse everybody will accept that you won't be able to get killed.
Finish your items with Force of Nature. The extra magic resist, life regeneration and the increased move speed gain you close to immortality.

Pulverize: Great ability to cancel any kind of ability the opponent could use, which last long time. Multiple stun surrounding Alistar. Nice to use in Team fights. Pulverize got a real long cool down so you should wait for a good situation.

Headbutt: Great in combination with Pulverize.Nice to bump enemies in range of your melee Team mates. Good to push Champions with AOE-Ultimates like Fiddlesticks with Crowstorm away of combat.

Triumphant Roar: A heal wich heals all surrounding allied Team members. Will reduce the cooldown of itself when an enemy gets killed in your range.

Unbreakable Will: Your free card to survive even a lot of damage. You will get close to immunity of damage. Must players use it while running away and die after its duration. If you initiate a team fight you should run in, activate your Unbreakable Will to avoid damage. And take the right second to stun surrounding opponent champions with Pulverize. It's real hard to say when to use it. After some time playing Alistar you while know when it's time to activate Unbreakable Will.

: Always prefer Flash with Alistar. Flash got so many abilities to use it. I always use Flash to initiate a teamfight, when my team waits for a good possibility to start.

: Great in combination with your Unbreakable Will. Nice for escaping.

: I prefer Cleanse on Alistar but Fortify is a nice summoner spell.

: A bad joke on Alistar. Why you got your Triumphant Roar. Heal is useless in end game and Triumphant Roar heals close to it.

: Never used Ghost on Alistar. Could be funny in combination with Cleanse. I prefer Flash, because you gain a surprising moment when using it.

Pros

• Great Support.
• High chance of survival.
• Great solo laning.
• Good options to cancel.
• Easy farming.
• Nice to initiate team fights.

Cons

• After using Pulverize and Headbutt real in a fix, because of long cooldowns.
• Can't get focus of opponents.
• Deals no damage when he is on his own.

Alistar is a great Tank at all. His posibilitys to cancel and stun targets are great. He is a great laning partner and won't steal kills easily.

I hope you enjoyed my Guide and give it a try for yourself.