This guide assumes you are playing a Warrior and want to or are willing to tank some dungeons on your way to 60. Maybe you have never tanked before, this is your first Classic Warrior, or you have tanked but in a different expansion where it just works differently.
It is over simplified and a lot of things that apply to late game/raid tanking are left out on purpose - because that type of tanking is different from what you are doing as a leveling dungeon tank.
Classic era tanking is not as clearly defined as it is in TBC and beyond - a lot of things can work depending on you, your party or raid, and your gear. If it gets the job done and the group had a good run - it was technically fine to do which includes 2-H tanking in good gear.
I’ll assume though that you are using a mix of quest rewards, dungeon drops, and most of the time you have a 1-H and a Shield equipped while tanking.
Spec:
It does not matter, they all work fine to tank all the way to 60.
In my opinion Arms is easiest overall because of Tactical Mastery and Anger Management.
5/5 Tactical Mastery is one of the best talents Warriors have and “Stance Dancing” doesn’t work well without it. Being able to keep 25 Rage when you switch stances is amazing. Without this talent every stance change starts over at 0 Rage.
Anger Management is good because you will get 1 Rage per 3 seconds in combat which ends up being a lot of Rage and it helps with having a good amount of Rage going from pull to pull. You can maintain your Rage by using Blood Rage to get into combat between pulls and Anger Management is a nice little boost.
I won’t talk about raid tank spec other than to say it is totally unnecessary unless you actually raid as a dedicated tank. Unlike every other iteration of WoW, any Warrior is a tank by playing a Warrior and being in Defensive Stance.
Defensive Stance:
You basically will be in Defensive Stance at all times. There is one good reason to leave Defensive Stance and that is to Intercept (level 30, requires Berserker Stance) a mob that is running straight at your healer and is out of range to Taunt.
While you are in Defensive Stance you take 10% less damage, do 10% less damage and do about 30% more threat (more with Defiance talent in Protection Tree).
Battle Stance and Berserker Stance both cause your damage to do 80% of normal threat.
This effectively means that in Defensive Stance you will comfortably hold threat on a mob with melee DPS doing as much as 30% more DPS. The number you care about is TPS (Threat Per Second), which is not exactly the same as DPS.
Really basic threat numbers:
In melee range another player has to pass your threat by 10% for the mob to switch to them. If the melee player is doing 105% of your threat, and you are in Defensive Stance and they are not, you will maintain threat on the mob.
Outside melee range that goes to 30% more. A caster DPS or a healer has to pass your threat by 30% to get threat on the mob.
It is really important that your caster DPS and healers are outside melee range. If you can move the mobs to create that distance, that is sometimes the easiest thing to do.
Parry hasting:
You want to face mobs away from the melee DPS. If the melee DPS are standing in front of the mob with you, each time their attack gets parried you get hit faster from the parry hasting. On a hard hitting mob or boss this can kill you or create panic very quickly.
In very simple terms, in Classic Wow every time an attack is parried, it immediately finishes the swing timer. This is true both for you, and mobs.
Of course the other reasons the melee DPS should stand behind the mob, or you turn the mob away from them is Cleave damage, and mobs can’t parry an attack from behind, meaning the melee DPS will do more damage.
Taunt, Mocking Blow and Challenging Shout:
Mocking Blow and Challenging Shout are fixates, in simple terms they basically put a debuff on the mob that forces them to attack you for 6 seconds. There is no copy of threat. When the 6 seconds expires the mob will go back to whomever is highest on the threat. What you hope for as a tank is that the 6 seconds (about 4 GCDs) is enough time to be on top and the mob stays with you. If you started with no or low threat on the mob, and another player has lots of threat, it is very unlikely that you will catch them and the mob will continue hitting them when the debuff expires. These two abilities work best early into combat when you can realistically establish #1 threat in about 4 GCDs.
There are times to use these even late into combat where the 6 seconds is enough to kill the mob(s) and it won’t matter that you are way behind a couple seconds from now. You’ll see these as you tank.
Taunt makes your threat the same as the player with the aggro/threat, meaning it does copy threat and forces the mob to attack you for a few seconds. In theory this gives you a couple GCDs to pass the other player’s threat so the mob sticks with you - but if not the mob will go right back to them.
Because Taunt copies threat, there are some more skillful ways you can use it to copy threat from someone who just got a big damage spike or burst. If you remember that it does this you’ll find some opportunities to use it.
Taunt mobs that are going for the healer as the top priority. Everyone else gets a Taunt if you can’t get mobs off them another way.
Threat basics
Essentially everything players do has a threat value when mobs are in combat. Gaining Mana/Energy/Rage, using abilities, healing, buffs, all of it increases the player’s threat to every mob in combat.
A classic example is if you pull a mob, and the healer puts a heal over time spell on you, the mobs will run straight to the healer. Why? Because all you did was shoot your bow, which probably missed - and the heal over time spell put the healer on the top of the threat table with all of the mobs in combat.
Threat is not the same as damage. Some of the highest threat abilities don’t do any, or not very much damage at all. It is great if you can contribute good DPS to the group while tanking, but it doesn’t necessarily mean you will have the threat.
Basic rotation/priority:
Before having your 31 point talent (level 40 or so).
Revenge → Sunder Armor → Battle Shout / Demoralizing Shout
Revenge is huge threat, and it procs off a Dodge, Parry or Block. This is the #1 reason to wear a Shield. Every time you press Shield Block, it gives you 75% chance to block the next attack, proc’ing Revenge.
If you use Revenge on the kill target every time it is up, you will probably hold threat until it is dead even if the DPS are focusing it down.
Sunder Armor is good threat, even though it doesn’t do any damage. In addition to the good threat, it increases the damage and the rage you will be getting from auto attacking the mob, and increases the damage your melee do as well. Alternate through the other mobs you have putting up Sunder Armor on them, switching back to the kill target to use Revenge.
Battle Shout and Demoralizing shout can basically be spammed to build up threat on mobs so they “stick to you” and can be helpful to gather mobs up on the pull. They will not be enough to hold threat on mobs if everyone is hitting something different - but it will give you a chance to pick them up and start using Revenge and Sunder to establish a solid threat lead. Remember to use both for initial “AOE” threat.
Thunderclap is not a good ability for “AOE tanking” in Classic. It becomes better in TBC. In Classic a DPS Warrior will use it to slow a boss’s swing timer to smooth damage on the tank, and that’s all about it is good for. It only hits 4 targets and does blah threat, in addition to requiring a swap to Battle Stance which further lowers the threat value of using it.
Heroic Strike is only good in situations of infinite Rage, where it becomes one of the most important thing to constantly have going. Outside of literally being unable to spend Rage fast enough which basically doesn’t happen in dungeons, it is not a good ability to use while tanking.
The reason is that Heroic Strike converts your next auto attack into a special attack - which means you lose the rage you would have gotten from the auto attack. This is terrible for dungeon tanking, and especially at lower levels before you have significant hit and crit - you will Rage starve yourself if you use it much to tank. Most Warriors can take it off their bars for tanking until it is time to raid.
Cleave on the other hand is a different story. If you find yourself with high rage, a good 2-H weapon and are tanking mobs that aren’t destroying you, sure go ahead and cleave them. Don’t over spend your Rage on Cleave, make sure you are also using Revenge (will only proc on Dodge and Parry without a Shield) and Sunder. Cleave can be used with a 1-H weapon, and you can certainly do that, but a 2-H weapon when possible is better. As a general rule don’t swap to a 2-H weapon when you are taking a lot of damage.
Once you have your 31 point talent, I would recommend you do Shield Slam/Mortal Strike/Bloodthirst (depends on which you have) → Revenge → Sunder.
You will get a feel for your Rage and how to make sure you always have enough to do these. Not over spending your Rage is probably the main thing that new Warrior tanks have trouble with at this level range.
Pulling:
Most of the time you should pull mobs, and not Charge into them.
By pulling mobs to a safe location you can avoid a lot of the screwups that happen like accidentally getting additional packs, having a patrol end up on top of you, and other noob traps that are in almost every dungeon.
Line of Sight pulls are essentially required when there is a caster in the pack. Caster mobs will not run to you if they can cast a spell on you. To make the mob run to you, you will have to break their line of sight so they have to run forward and get a clear path to cast on you.
Have your party wait at a nice corner somewhere, go up pull the mobs (with a gun/bow/thrown/crossbow), run back to the corner with your party and wait for them to come to you. Smash the kill target in the face, and do your tank thing.
Marking:
I recommend marking whatever needs to die first with a 
. I might even move this around as mobs die to focus them down in order of most to least dangerous.
You can’t get mad at the DPS attacking random stuff if you don’t tell them what to attack.
Some of the common meanings of raid markers (“lucky charms”).
“Skull” - kill this first
“X” - second mob to die
“Blue Square” - third mob to die or freezing trap
“Crescent Moon” - sheep
“Star” - Sap
“Yellow Circle” - Mind Control / Shackle
“Purple Diamond” - Banish
“Green Triangle” - Root
Healer Mana:
Keep an eye on your healer’s mana. If you have several mana user DPS in the group, watch theirs as well. Smooth is fast, and healers and mana user DPS having mana to do their job is going to make everything a lot smoother.
You can stay in combat to keep your rage up a few different ways. Blood Rage will help you keep rage while they drink with Anger Management. Killing critters between packs helps, think of them as nice little Rage pots. Find ways to keep your Rage going and let the healers drink.
They don’t need to be out of combat to finish drinking, but they do need to be out of combat to start drinking. Use that to you advantage if it feels right.