Hello guys,i made this guide so i can help out the players who want to learn this champion. This guide is in construction but i will update it constantly.
I am a Gold 3 player at the moment on the EUW server,and i have 165k mastery points on Alistar

As i said,i'll explain what items you can buy in each scenario.

Mobility boots : Great buy if you are ahead,you can catch up on enemies or you can roam.

Tabi Boots : Buy this if they have a full ad team and you are losing or if you are vs a fed autoattacker,as his/her basic attacks do less damage to you.

Mercury's Threads : Buy this if you are behind and if there's a heavy ap team or if they have lots of cc.

Face of the Mountain : Great item early game if you are ahead. It gives you an extra shield,which may save an ally or "secures" an assist.

Eye of the Equinox : Ok item if you are losing. The purpose of this item is that you can profit from an extra slot,so you can become even tankier,aswell as 1 extra ward early.

Locket of the Iron Solari : This item can be a lifesaver in teamfights due to its shield. Buy this early if there is a fed ap in the enemy team. You can also buy this early if you are behind,because of the shield and the combined stats. (Mr and Armor)

Frozen Heart : Buy this early if there is/are fed autoattack based enemies. The armor and extra mana please Alistar very well. Also buy first Glacial Shroud.

Righteous Glory : Good item if you are ahead,you activate it and you gain some movement speed while running to the enemies and when you are near to them they get slowed.

Randuin's Omen : Buy this if you are against crit based champions. Its active slows them when you are near them (Alistar w->q combo),giving you one extra CC.

Knight's Vow : I usually buy this if my ADC falls behind so i can soak some damage if my ADC is focused. This can be a lifesaver.

Zeke's Convergence : Buy this if your ADC is ahead. When you ult your ADC gets empowered attacks so he/she does even more damage. Also after you cast your ult the nearby enemies are slowed so this also gives you a bonus CC.

Thornmail : I usually buy this vs a heavy AD comp or if there is a fed AD enemy who builds lifesteal. Buy Bramble Vest first.

Spirit Visage : I like to buy this against a heavy AP based team. The stats are nice and the extra healing boosts the passive.

Gargoyle Stoneplate : This item is great if you are the only tank on your team and you want to soak as much damage as you can. Activate it when you start to recieve damage.

Ruby Sightstone: Buy this if you bought face of the Mountain. Buy last.

Mikael's Crucible : Buy this against heavy CC teamcomps and use it after your carry gets cc-ed.

Banshee's Veil : You can buy this vs champions like Veigar,Khartus.

Adaptive helm : Great item vs enemies who use a low cd spell or dps. (Cassio,Singed,Teemo,Swain)

Abyssal Mask : This item give you nice resistances. You can buy this if you have a fed ap ally,so he/she can do 10% more damage.

Warmog's Armor : You won't buy it too often because there are other items you can buy and give you bonus resistances. Warmong just gives you some health,cdr and health regen. Thing is: You'll be in the middle of the fights and most likely people will attack you,the passive will most of the time be on cooldown during the teamfights so you can't really use this item fully. If you still want to buy it,buy it with Spirit Visage.

When to take ignite vs when to take exhaust.
Is there an assasin on the enemy team? Get exhaust. Otherwise get ignite.
Ignite helps a lot with the early kills,basically feeding your ADC and preparing him to carry.
Exhaust helps in teamfights vs an assasin. They will try to kill your carry,so you have to counter that.

Pros:

Amazing tank,especially with ult. You can activate it and go sleep for 8 hours,you still won't die by the time you wake up.
Amazing sustain early game from the passive. Lost a trade? Just let your ADC farm and watch how your health goes back up.
Easy to play. Honestly now,you got 2 point and click abilities and only a basic combo to learn,pretty brainless.
Amazing engager/Peeler. You got 2 cc and a displacement.

Cons:

Long cooldowns. After your combo you got only autoattacks left to help your allies for a while.
Somehow imobile.
They can run/dash away from your e while is charging so sometimes you can't hit the stun.
Can lose fights if he misses his combo.

Early game : Let your ADC farm and engage whenever you got an upper hand.

Minion advantage : When you got more minions than they do,so they take more aggro than you do if they fight back.
Level advantage : You got higher level and your spells do more damage.
Item advantage : You got more items than they do,so you tank more/do more damage.

Try to roam if you backed and you aren't really needed in lane for a minute. You can go midlane to set up a kill for your ally. Also watch the map often and if a skirmish nearby starts make sure you and your adc follow up.

When to back :

Best back : After your lane is pushed under their tower.
Ok back : Cannon minion wave.

Tower vs Dragon after a successful gank. Are you low hp? Tower. Have you got hp? Dragon.
Also if the first tower wasn't distroyed yet,go for the tower.

Towerdive : When you towerdive the most important thing you need to know is where the enemy jungler is. You win a trade on bot,you kill one of those two botlaners and you want to kill the other botlaner too. Ping that you want to towerdive,then proceed to W -> Q -> E. You can activate ult after you got CC-ed or if the enemy doesn't have a CC you can activate it when the tower starts attacking you.

Warding early: Control ward in the bush in the river,next to your lane.
Are you push under your tower? Normal ward in the tribush next to your tower.
Do you want to dive them under their tower? Normal ward in the tribush next to their tower.
If you notice that the enemy support constantly goes in a ward on your lane,ward it. Also,if you have an extra ward and you don't need it for something else,ward dragon.

Midgame and lategame

You want either to engage on the biggest threat or stay back and peel for your carry. If you are the only engager and ahead,ingage. If there is another engager on your team,peel. If you are behind,peel. If there is an enemy who heals (Swain,Garen,Mundo,Soraka) its advised to ignite that person during the teamfights,so you reduce their healing.

Warding midgame:important objectives such as dragon or baron if up,control ward in the small bush in the river,next to dragon or baron,and if not ward their blue/red. Sweep always when it's up.

Also always try to assist when you take an objective because this can give you a higher rank at the end of the game.

General combo : Press w when you are in range and then hold q,so you can always do your combo.
If you are close to them you can walk to them then q when you are in melee range,because if you are close to them and you w,you miss your combo.
Also : Q -> Walk behind him/her ->W him/her to your allies OR Q -> Flash behind -> W
Activate ult when they focus you or after you get CC-ed.
You can also q them then push them into a wall,so they get stunned for a bit when they hit the wall.

Let me explain,this works so well,especially if you have AA champions on you team. We rush it early so in the midgame we can already have it completed. This works best with Stoneborn Pact keystone. The strategy is that our movement impairing abilities (Alistar Q,W,E) put a mark on the ones we attack. If the allies attack the same person/people,they heal for a bit.We don't look here for big heals,we just want a heal that activates Ardent's passive. Also,shields work too so if we build Face of the Mountain or Locket of the Iron Solari,we can give everyone a shield with a well activated Locket.
Overall i think Ardent can help with Alistar's low damage output. Not only that you all get an attack speed buff,but your team's basic attacks do 25 more damage and heals up to 35 HP per basic.

Thank you for reading my guide. If you have any suggestions or questions,you can write a comment and i will reply as soon as i can.