Classic - Deadmines

[vorticone]

Long term warrior tank here, ever since vanilla WoW until some way into Ulduar. So I think I can give you some advice.

If you tank as a warrior, you need a lot of micro-management. This is especially true at the low levels where you do not have every ability available to you yet. The real problem you are facing is, you only have active threat abilities, nothing passive. You basically have to hit and sunder each and every mob you fight.
Once you get Thunderclap it gets easier, but be aware Thunderclap does not hit all mobs in range but is capped. The higher level you are, the easier it will get. You get a lot of useful pulling and threat tools rather late in the talent tree or while leveling.

If you want to keep your warrior as the tank, I can recommend a few things. First of all, play him as you main character as he needs a lot of attention.
Use the Sunder Armor glyph as soon as possible (it makes your Sunders hit two mobs instead of one unless they changed it), and then a glyph of Thunderclap. These abilities will be your main threat abilities for a while.
Grab a shield spike if possible for your shield, as it counts to your threat when mobs hit you.
Make a mouseover macro for your Sunder Armor skill, so you do not have to re-target every second.

And the most important rule: have one main threat target. Keep pushing out threat on this target and have your dps go nuts on it. Do not try to AE (yet), it will be more or less impossible to hold aggro on a bunch of mobs while playing more than just the warrior.

Warriors are probably the tanking class that needs most attention to details. If you want to go an easier way, switch to paladin or druid. I love warriors, played them forever. But for my multiboxing I chose something simpler so I can divide my attention to all of my characters.
And if you stick with your warrior, good luck. Learn how to pull, learn how to line-of-sight, learn how to quickly change targets. Be aware of your surroundings.
Best to learn to tank while not multiboxing though.