Ok, so this is more a guide to leveling that tanking, but it is in regards to being a tank. Many Warriors got DPS to level. Personally I do not agree with this. Early on, it may be easier, but I think it truely becomes EZ mode to level as Prot as you approach the higher levels (most notably hitting the 60s). So without further ado, here is your guide.
Instances: Yes, instances are your friends for many reasons. They may have a quest in there for you that will net you some nice XP. Upgrades are another huge reason. The better your gear, the easier it will be to level. You get a lot of XP from running and instance and you get the experience of tanking as well. That being said, there arent always people looking for a group in Outlands etc. My suggestion is to put yourself in Looking For Group and go out and quest.
Questing: Questing is awesome for a number of reasons. You make lots of money for one thing. Often times you will find some decent upgrades as well. While out questing, I suggest pieces that are high in Stamina and Strength. Avoidance is also very nice, but high healts and strength generally do the trick. Depending on level and what kind of quest you are doing, you will use different abilities. If you are going to AOE grind the mobs you need for a quest, gather them up, demo shout, thunderclap, and shockwave (given that you are the appropriate level). Use revenge and shield slam every chance you get as they are your two big damage dealers.
Keeping Yourself Healthy: Any Warrior worth their shield should have First Aid as high as you can get it. Always have a lot of bandages on you. You get cloth off of the mobs you kill and can turn these into more bandages. Dont be afraid to use them. bandages can often save a nice chunk of time as opposed to eating, Another helpful proffesion is cooking. If you have cooking on one toon, that is usually sufficient. Save the meat you pick up while questing and have it cooked up for delicious buff food. You would be surprised how much it helps out. Always have some healing potions on you as well. You never no when you are going to bite off more than you can chew and you always want to be prepared.
Leveling as a Warrior is not as bad as it once was. Some will remember how it was to level in Vanilla WoW as a Warrior. If you pulled more than one mob you were usually screwed. That is no longer the case, and leveling as Warrior is pretty simple.
So, the rules are: Get an instance if you can, Quest your ass off, and keep yourself alive. You will have a max level tank in no time and be ready to take Heroics by the throat.