There's advice everywhere for specific things, but you'll only ever understand through trial and error. Sounds like the first thing you need to do is learn to tank, which a lot of people have to do coming from single boxing.

Tanking is not just about hitting the right buttons at the right time. Most of it is knowing exactly what is going on at all times and what is going to happen and how to control it. Learning how to pull ranged mobs, how to position, how to be patient, determining patrol paths, controlling runners, aggro building.

Personally, I don't like warriors. I feel I get a lot more out of a paladin tank than a warrior. And most people would recommend a paladin to someone starting out. If you're dead-set on a warrior, though, I'd suggest waiting a few more levels until you start getting the tools that will allow you to tank.

As far as specifics :

Defias Evokers/ranged mobs - Move your team far into a safe area. Attack the mob from range {gun/bow} and run your tank back, using a wall, rock, doorway, whatever you can to force line of site issues for the mob so that they are forced to come into your melee range.

Hunter-type mobs are generally easier as you can deadzone them without having to use the environment for line of sight.

Casters do not have a deadzone and need to be forced to the location you want them by either forcing them to melee {silences, mana drained}, long-pulling {purposely running out of their casting range and past your group until they are in range of your team, then running back up on them} or removing their line of sight {putting something between you and them that they can't cast through, which does not always work}.

For dealing with the flamestrikes in particular, you should leave a set of movement keys that pass to your slaves. Most people use the arrow keys. I also have Q and E passed to my slaves so they can strafe.

Patrols - Be patient. Watch for them before you pull anything. See where they go and how long it takes them to get back. Then pull the set of mobs you want AWAY. Do not tank mobs where they stand. This will save you a lot of headaches with runners and adds.

Seeing everything - This is just something you have to learn. As other things become second nature you will have more time to keep an eye out on other things going on around you. Making use of areas you've already cleared will help keep things in front of you and buy you more time to see what's coming.

Warrior tanking - I have very, very little experience with warriors outside of healing them, but I can't tell you how many times I have heard them screaming at DPS "WAIT FOR 5 SUNDERS!!!!" While that's not the warrior of today, you do still need to build aggro before you have your team engage. I'm guessing you don't have thunderclap yet, but you will need that for AOE threat. Another means of gaining threat on multiple mobs is sundering. Pull the group, sunder the first mob, tab/target the next, sunder, target the next, sunder. THEN start DPS. The autoattack + sunder should give you more than enough of an aggro lead over your team.

If your priest is ripping aggro because of heals or shields, then don't precast. Until you have the tools to snatch AOE aggro {TClap, Cleave, tab/sunders} you'll have to hold heals back as much as DPS. This will only be a temporary problem. Eventually you will get the feel for it and you can precast without it meaning certain death for your priest.