There are plenty of Alistar guides out there, this is a guide for a pure tank/early support type Alistar. Your role will be to pick out squishies and feed them to your teammates. If you like to play to get kills this will not be your build. If you get satisfaction out of lots of assists and picking people out of the crowd, this Alistar is for you. Your purpose is to help your laning partner in early/mid game and then to be the tank and CC in late game. A good lane pair with Alistar will get fed several kills in early game.

The runes that I use are not set in stone and can change on your individual preferences. I choose magic resist for marks to help in early/mid game survivability. Getting the armor items I usualy get first requries more magic resist in runes. Same goes for Seal of Fortitude, having an extra 48 health can help greatly early game. Quint of Swiftness are obviously for that extra speed boost and I pick up cooldown blues for obvious reasons. Part of the goal is to spam Triumphant Roar to A. keep your laning partner healed up and in lane, and B. (at times) to keep your passive up against minions. Runes can really be moved around depending on your play style and what you are looking to achieve. Subsituting more mana regeneration is a very reasonable alternative for any rune set. The Seals of Fortitude could easily be switched out for Replenishment for better mana regen or Focus for reduced cooldowns, although I have never used Seals of Focus.

Masteries are simple, a tank build with some mana in for good measure. Again, nothing is set in stone. Building more utility and limiting defence would allow for more mana regeneration and also getting the gold per 5 and starting gold for some more support items at the start.

Being as you will likely be bottom lane with you're ranged AD teamate I start building towards Philosopher Stone first foremost, starting with the Farerie Charm and health pots and two wards. This is somewhat dependent on what you are looking to bring first. You can subsitte health for mana pots, but this may mean getting heal at level 2 if you run into problems. Philosopher Stone is a great support items, especially for Alistar who uses a lot of mana if needing to heal often, plus there is the gold per 5 aspect which is in high demand. Second is movement speed which build into Mobility Boots for speed to get into team fights and pull guys out. Some prefer Boots, but I have always found Mobility Boots to be a better alternative. Other choices are Merc or Ninja for armor/magic resist. After that is Heart of Gold for more health and more gold generation. After this you should focus on coold down reduction (CDR) items. I go straight to Glacial Shroud because you should be facing another attack damage carry in your lane and it gives mana and 15% CDR.

After that it is really up to you and time/team dependent. Spirit Visage is a fantastic item for Alistar to give him extra CDR, some health, but more importantly extra healing on himself for an added bonus; start with Kindlegem. Finishing Frozen Heart is a good way to get an item to counteract the enemy carries and also tops off your 40% CDR. Locket of the Iron Solari is a new item that builds out of Heart of Gold and it is fantastic for teamfights and is a much nicer to build into rather than Randuin's Omen. The final Magic Resist item is really up to you, Force of Nature is a great item, gives extra health regen, and movement speed; but I also like getting Abbysal Scepter if no one on the team has it, extra ability power is an adde bonus on top of the aura effect it gives off and the magic resist. The final item is really up to you, many people will build philosopher stone into Shurelya's for the movement bonus, but your CDR is maxed by this point so I usually go into Giant's Belt and to Wormogs Armor or for a CC heavy team into Eleias Miracle

If you are having issues with one specific champ, then switch up your build and get some magic resist or some armor, whatever is required. If you are always building the same items in the same order ever game, you are not paying attention to what is going on, you are just going through the motions. If you are in teamfights that require lots of CC, then get a cooldown reduction and mana regen item earlier in the game. Frozen heart is great for some defence and cooldown reduction if you need it earlier.

Summoner Spells are easy choices; Flash first and foremost, any Alistar not using Flash is either not high enough level, or has no idea how usefull it is. Using flash to get to a champion in order to combo is essenctial and a deadly tool. Not having it, is not using Alistar to his fullest. I use Clairvoance second to show areas of the map, it's a support spell and you should pick it up since no one else whill. Ghost is an option for chasing people down and also to get out of trouble. Teleport could be a reasonable alternative if you want to wait to get your boots till later. Other reasonable alternatives are clarity if you have mana issues and exhaust if you want to be able to slow champs/limit their attack, but your lane partner should have it, Heal is the final spell that is acceptable, but I dont take it for my more tanky build.

Skill priority is simply R>E>Q>W, but starting with Pulverize and then Headbutt to get your combo. After getting them first go Triumphant Roar to heal your lane partner and for your passive spamming, it will always be the cheapest option for Alistars passive, even as you hit level 5 with it. Pulverize and Headbutt are your two hit combo that is essential to playing Alistar. If you are not familiar with this already then get into some bot games and use Pulverize (Q) to knock them in the air, get to their other side and Headbutt (W) them back into your teammates, which is when the real damage takes place. Pulverize and Headbutt do not need to do lots of damage, that is not the point, they just need to move the enemy champions into vulnerable positions. You also do not want to spend unnecessary mana on slightly more damage with the combo, so keeping those levels down till mid/end game is more efficient. This is why most of your early levels will be put into Triumphant Roar. TR gives very good healing abilities to yourself and also minions and other teammates in your area. TR also triggers your passive which will deal damage to surrounding minions and champions. Using TR to spam your passive against minions is an effective way to push your lane in early game. It is not the fastest lane pushing method, but it’s fairly mana cheap and keeps you and your teams health up, staying in lane longer and making ganks harder. TR will also keep you and your partner in lane for a very long time.

A great tactic if your lane is pushed is to just sit in the lower bush and wait for the enemy champs to come out far enough from the turret to Headbutt them into the corner of the wall near river and then Pulverize them while your lane partner attacks and hopefully kills. This will leave you in the bush to keep get mana back faster and also to be prepared to defend against a gank from the jungler. it may also cause the enemy suppor to waste wards on that bush, which is when you pick up a pink ward to simply take it out and have vision on that bush for 2 minutes.

Unbreakable Will is your ultimate and should be used when you know you are about to take a lot of damage, not when you have only 100hp left and are surrounded. UW is also useful on turrets if you need to tank them, hit champions out of their range, or just want the extra damage output to take a turret out quickly. Don't be afraid to use UW when a turret is nearly down and no one is around.

Pros:
Great health and resists survivability
Great team support player
Ability to pick out the squishies from the back.
Cheap healing and passive damage

Cons:
No real damage dealing
Mana can be a problem early, but it can be mitigated by playing smart.

This can be a nice build for a supportish tank. If you don't like it then change it. Cooldowns is your main focus. Alistar has arguably one of the best tank abilities in the game with Unbreakable Will, use it to your advantage and don’t be afraid to use it on a tower to tank it and for the added damage output. Remember to always have fun, it's only a game.