I quit WoW about 10 months ago and have just started back up so please excuse me if this advice is out of data or doesn't pertain to prot warrs anymore-
With that disclaimer in place I have to say that the most common mistake I used to see when training a new tank was a lack of situational awareness. While this is a skill used by almost any class in a raid and I feel that tanking requires a level of awareness of your global environment well above that of any other class.
Because you are not encountering end-game content yet and you don't have all of the tools that you will have at a higher level I would recommend practicing being aware of your environment. You have to be able to pay attention to everything that is happening around you while continuing to keep threat on whatever your current target is. As you progress to higher levels there is more and more to pay attention to so it is a good idea to start becoming aware of it now.
Watch your health, watch your rage, watch the health of your party members, watch your threat on not just your current target but every target your party has currently engaged. Watch CCs so you can know when they break and you have to constantly be ready to pick them up if the player in charge of CCing them forgets to resheep/stun/whatever (this last one you probably wont have to do at lower levels).
Other then situational awareness there is all the usual advice- get good addons that help, commit your rotation/priority to the point where it is just a reflex. Be open to learning new things and know that the difficulty of what you are doing ramps up as you get higher in level and see new content.