This is common to people who are new to instances. This applies to people whose first instance is at level 19 in Deadmines or at level 70 in Steamvault. In instances your view is limited because you're in a mine shaft or a ruined city or wherever. Couple that with the fact that you're typically fighting multiple mobs and that everything just seems to move faster than you're used to and it's no wonder you can't keep track of everything. This is ok, for now. This is one of the reasons that you want to tell your group that you haven't run instances a lot. However, it is one of the reasons that tanking classes can be difficult as a first toon because, as a tank, you're ultimately responsible for keeping track of all the mobs running around.
There's no real trick to fixing this, though. You just have to run more instances. As you do you'll find that it's easier to see what, exactly, is going on and what needs your attention and what someone else is dealing with. The more you run them the more you'll see patterns emerge and the more accustomed you'll be to the pace.
If you're a level 70, it may be wise to run some of the lower level instances first like Mana Tombs or Sethekk Halls. This way it's going to be a little more relaxed for you as you'll out level the mobs by a bit. (though this may be tough as a DPS class since you're going to be doing more damage. Watch your Threat). Just make sure your group knows that you're not running them through it, that you're new to this and are trying to learn.
It depends on your role in the group (and sometimes on the encounter, but we won't get into that).
As the tank, you can pretty much stand where you want, everyone else will adjust to you. That said, you want to pick a spot that's pretty far away from other mobs so you don't accidentally pull them, too. It's also helpful on many boss fights if you can turn the Boss away from the rest of the group. You want to watch out for Crowd Control as well if you're using any abilities that hit multiple mobs (like Thunderclap or consecrate or even swipe). If you see that you're standing next to a sheep you want to move before you use these abilities. With practice and experience you'll get a feel for how wide your range is on those abilities and will be able to easily adjust to them. In the beginning move pretty far away.
As the healer you're going to want to stand away from the group to avoid any damage, including AoE but withing healing range of all members of the group. Be aware of what's around you in case you need to move, though. You don't want to pull adds.
You want to be standing behind the mob you're attacking, even if you're not a rogue. Why? Because if you stand behind the mob they can't block your attacks. This means that more of your attacks will land on the target and damage it. It can also help you avoid frontal cone AoE attacks or cleaves that any mobs may have.
In some instances you're going to need to spread out to limit the damage of some boss abilities. You should ask your group about boss fights (before they begin) to see if this is the case or of there's anything else you should know. If it's not, and for now, you should stand next to the healer. There are 2 reasons for this. 1) You know you'll be in range and the healer won't have to adjust into a ba position to heal you if needed. 2) and more importantly, you'll know when the healer is taking damage from a mob. If you're standing next to the healer it should be pretty easy to see if a mob peels off the group and runs to the healer. Chances are you'll be at about a 30 yard range so this will also give you a little time to react. You want to whatever you can (trap, immobilize, or high damage spells) to make sure the mob doesn't hit the healer. Protect your healer at all costs, even if it means killing yourself doing so.
The Voidwalker can be a great pet for soloing. He's got high armor and can tank a lot of enemies. But in a group you already have a tank. There is almost always a better pet for an instance than a voidwalker. I'm hedging there, really. In any instance or raid run I've ever been in I have never thought to myself "Wow, this would be so much easier with a voidwalker".
His damage is laughable. Compared to your actual tank his mitigation and avoidance are pretty crappy. Compare that to what your other minions bring to the table:
There are numerous reasons for this beyond the fact that it's just not needed. One is, from your point of view, having growl on will needlessly eat up a pet's focus that can be better used for DPSing the target.
However most of the reasons come from the tank/healer's point of view.
Tanks like to have aggro. It makes them comfortable. They spend much of their time focusing on the main mob and the other mobs to make sure that they're holding on to all of them. It's very frustrating to be fighting with a Felguard or a hunter's pet for that aggro. Unless the tank specifically asks for it, they really don't need your help with the mobs. EVen if there are multiple mobs, they don't need your help. The three tanking classes have abilities that allow them to (to a greater or lesser degree) hit more than one mob at a time so holding aggro on multiple mobs really isn't that difficult.
Tanking is a control freak scenario, and with good reason. If a mob peels off the group and eats the healer it's always the tank's fault. Even if someone's boar growled him off and couldn't hold the mob against the healing aggro. It's still the tank's fault. Having your pet taunting mobs off introduces a variable that the tank can't control. Having that element of surprise introduced makes tanks very uncomfortable and it can also be detrimental to the group.
As a tank you make split-second decisions on what abilities to use, when. There generally isn't a whole lot of time to look and see what, exactly, is going on. So if your pet taunts the mob and it turns away from the tank all the tank sees is the mob not paying attention to them anymore. Generally when that happens, it means that the mob is going after the healer so the tank will immediately blow an ability (a taunt or stun or whatever) to corral that mob back and get aggro. However, since it was just going after your pet, this means that the tank has now blown the cooldown on a critical ability that may be legitimately needed later to protect the healer or prevent a wipe.
Consider, too, that a decent to good tank has put some effort into gearing themselves for the job. This means that they're pretty much going to be geared miles better for the job than your pet. This means they have more mitigation than your pet. And this means that it's actually easier on the healer to be able to focus on one tank and keep them topped off than to have to split focus between one well-geared tank and one poorly-geared one (your pet or minion).
Having an off-tank, whether it's a pet or a player, is a tactic, but there are almost always better tactics to use.
That said, even if you have growl off you should have it hotkeyed in case something goes wrong. While it is the tank's job to protect the healer, it's yours too and a growl/intervene if something peels off and goes after the healer can endear you to any group. The tank will likely be right behind your pet to pick the mob back up, but having that couple second buffer can really make a difference. In that scenario a taunt or an intervene is absolutely ok.