Hello LoL community, players mainly!
Here are the formalities!
This is my very first guide, and I hope everyone like it...
I made this guide on Alistar because i started playing him a lot recently and well... He suits my play style! xD
The reason for that is that I love to annoy enemies with a Headbutt and I love when they decide to chase a low hp champion (me) while they have the rest of my team after them ^^
It happens a lot, Alistar has heal, some CC and an awesome shield, why not focus the tank?
They shouldn't because it's Ali...

Oh, i also need to mention, this guide is not complete (of courrrse), so i will update it when i have time to spend on it xp

Flash: It's a must!!
Better not take Risks, you are the support but no need to feed, and if the enemy have used his flash before and is on cd, it's nice to flash past him just to headbutt him and get a kill for your adc.

Teleport: Not everybody likes it when you get teleport, but what if both me and the adc need to go base while the enemy adc is charging on tower with jungler or support?
This spell can make you win the game just by protecting towers, this spell allows you to teleport to a ward for ganking, you can with this assist top lane in 3 sec. why not use it?

Optional:
Exhaust, it's a nice spell if you really need to secure the kills, and it might help you land a proper Headbutt without the risk of saving the enemy. I highly recommend it if you are not familiar with Alistar.

Barrier, only if you are afraid to die, it's a must for howling abyss but not that great on summoner's rift in my opinion, at least not for a support tank...

Clairvoyance, if you really work well with your adc he wont need to shot the target from a mile away... It's nice to see if the drake and baron nashor are safe but it's not like you are best friends with them, and you will buy wards so use them instead. This spell can be good for Lux for example... (Dat ult... pfff)

Clarity, Cleanse, Revive, use it if you wanna troll with your own team, these spells are a waste for a champion like this one and i don't even know why people use Revive... for anyone! Learn to stop damn suicides and tower dives with low hp and you won't need it.

As for other spells, take Ignite if you want to ks (duh, no Ignite for Alistar) take Heal if you feel like it, although you don't need it (aram only imo) and no Ghost for you, Hecarim needs ghost but alistar don't, i like ghost but not for him. Choose a better spell and don't troll with your team, only with enemy team.

So, why do i start with those items?

Faerie Charm: I always buy two. Alistar drains mana like hell, he needs this to stay with his adc on the lane and assist him for the longest time possible. If you get really low hp and low on mana try to give a final push, go base, buy something even if only a ward (don't buy pots!) and now if you got Teleport you are a good bull and a good support as you will save your adc from being slained. Or not...

Mana Manipulator &
Philosopher's Stone: I told you why above, get the manipulator when you go base for the first time, (your Adc will like it too) if possible get the stone too, if not, wards.

Kage's lucky pick: I love gold, don't you? You will need this for item completion anyway so get it soon for as much gold as possible, that's why you should have a tome in consideration when you go base in the early game, it will depend on how your team is going, if it's bad get wards instead, it's incredible how the vision can affect the game when we have a blind pro champion ( Lee Sin)... Even the blind need wards for something.. xD

Sheen: More damage, if your Adc has trouble dealing the damage, you give will be kind and do damage enough to get him feeded.

Glacial Shroud: If you are having trouble against the enemy Adc, get this instead of Sheen

Emblem of valor: I only get this late game, it is in the optional list for me, but i needed to fill the complete build...

Up until now my games with Alistar go so well that i never finish the build :'( I JUST WIN BEFORE BEING ABLE TO DO SO!!!!!!!!!! (Such a sad fact...)

Anyway, I always finish at least with:

Mobility Boots: Love this item because I like to move it move it!
Eleisa's Miracle: Love it, if you have the gold, it's worth for almost every champion
Iceborn Gauntlet: Same as above, and no stats get wasted :p

Sometimes i have the time to complete:
Shard of True Ice: I think i never remember to use the passive but if you do, you do better than me xD
Spirit Visage: Awesome when they chase me and my heal is op thanks to this toy :3
WARDS!!!!!!! As much as i can!!!

My personal advice (thanks to Laggermeister and Sirell for pointing out this):
While some people think this item is a must i don't, and i know people will comment this negatively, but regarding the map awarness, my advice is:
Don't buy a Sightstone / Ruby Sightstone.
Why?
Because they are expensive!

For the early game I don't find it viable to spend 950 gold on an item that gives you a little health (180) and two active wards for each time you return base.
You are not supposed to go base often, you are supposed to stay in lane the most time possible, help your adc farm, poke the enemy adc with Headbutt and if your pal needs to go to base, defend the tower, wait until he arrives, if you can, stay in lane, if you can't go base then, get what you need (more wards/upgrade items if possible) and teleport back to bot lane. (If you have a ward on a bush past your enemy, Teleport there so you can Headbutt them into your tower range and get a kill for your adc.)

So, in stead of the Sightstone I prefer to rush the Mana Manipulator, wich is very cheap and also sustain adc's mana (thanks riot staff), get 5 wards wich will cost 375 gold and do the Philosopher's stone if you haven't it yet.
If you do, buy an Amplifying Tome if you haven't your Kage's Lucky Pick, or you might otherwise get a Sapphire Crystal for Glacial Shroud or Sheen.

Sorry, it's late, i spent two hours already doing this guide and this topic is well... not so important imo...

I recommend the masteries and runes i use because it enables you to adapt, if you need to be tank, yes you can, if you need to do damage you got at least some armor penetration from the masteries i chose, so get Ap, and concerning the 21 points in utility, they will help both early mid and late game thanks to movement, mana regen and gold.

Regarding the Runes, they are tank runes, i love the movement on quint and will never replace that.
I have to mention that although i don't use those Mark Runes (I have attack speed runes, i'm cheap so i won't buy more right now xD), I highly recommend the scaling Ap runes on Alistar because with this build, the magic resist from items will come late and it's a great back up in case the enemy caster is having a good time. If i weren't so cheap that's what i would buy (lol xD).

So, again, I am sorry, i won't list the masteries I use for Alistar. Mobafire already had the kindness to allow me to use a chart that i could fill... It's above, look for it :P

Well, this will be my conclusion before I enter sleeping mode...

So,
When playing Alistar as a support, DON'T KILL!!!
Unless the enemy is pissing you off and you really want to punish the @£#*§ (i don't know what this means...), but seriously, don't do ks, get kills if no one else can or if your adc is bad and you need to carry a bit.

Oh, if your adc is a Skarner, Garen, Irelia, Nocturne (those champions that aren't adc but people thinks they are pro and they are absolutely sure these are adc) don't bother with letting them farm, do the mana regen items and get wards (not everyone needs to pay for one's foolishness), but you can go more offensive and kill... I would rely more in Annie adc than Skarner (I did a custom and played Annie adc once for fun and it can get viable, but only late game, unless you have ad runes xDDD)

Again, BUY WARDS, as much as you can!!!
Once I believed that Alistar was build dependent... I was sooo wrong...

You don't need a complete build, you need to get your teammates kills when playing alistar, and be careful not to kill and not to save enemies... That's why you buy wards.
Vision on map helps everyone, although there are some people who start looking at the opposite lane when they recall... NEVER DO THAT PLEASE!!! You can feed so much by doing that it hurts :s (dat drama...)

Spam your Triumphant Roar as much as you can, with the mana regen you get from items and if on top of that you have the luck of getting blue buff, you can spam it as much as you want. (don't waste mana though, unless you want to troll and heal when you are full hp in front of enemy team :p)

Train your Headbutt!!! It is so awesome when well used, so fun when well used, i love when the saviour of enemy team, King Darius appears out of jungle with a 16-5 score, he prepares to enter a 2 on 2 fight, pull my mid laner, readies his ulti... headbutt! NOOOOO! You no kill my teammate.
Freaking awesome ^^
(this actually happened on a match 2 days ago and i Headbutted that feeded Darius into the jungle again, i was singing "I'm trollin, trollin, and i hope you like trollin too".

Last but not least:
Use your ultimate, Unbreakable Will well.
This is not a very tanky build, use it to tower dive, get in teamfights or survival issues. I recommend that you don't wait until your health drops too low to use it, use it when you are a bit above half health, i like to make the enemies focus me first. They will do damage quickly but they will broke their nose against an Unbreakable wall because of that.

This ultimate is a real brake for enemy damage, but you need to get the right time, if you are too high hp they won't do much damage, thus, they will focus other teammates, if too low hp, you will die because you used it too late... Let them think they are hurting you so that you can turn the tide with that wall :)

And that's all for this guide, again, i hope you enjoy it. Thank you for thumbs up!

or not... :p