Okay Dave, now that you've chosen a pet of sorts (remember you can have up to 5 now), its time to learn how to use him (or her - if your pet is a girl).
With the new expansion several things have changed. All pets now have a growl (which they could have before unless you were being really silly and didn't train it). All pets have a fear, they all have a focus dump (what else would you do with all of that focus coming out your ears), and last but certainly not least, they have their nifty talent based on their pet family.
You'll also get several pet abilities when use up your pets talents in his (or her) given pet tree. Your tenacity pets will have lots of cool stuff that will help them hold aggro better, like taunt. They also have some intervening abilities that can save your tank, or a clothie's butt if needs be. Ferocity pets have some neat things that will help them stay alive longer, like Lick Your Wounds, and a spell that will Rez them with full health (Heart of the Phoenix) which currently seems to be bugged.
Speaking of bugs, sometimes your pet will automatically turn on the abilities. You know when their spell glows, that means its suppose to go off whenever the cool down is up and your pet has enough focus. Well, some abilities were not meant to go together. For example: the pet's fear spell was not meant to be put up the same time as growl. You have an ability that increases your pet's aggro and one that decreases your pet's aggro. What generally happens is that the mob gets so confused as to weather or not he wants to attack your pet that he gets extra angry and he takes it out on you. If you have this problem generally you be thinking your pet is stupid and can't seem to hold the aggro to save your life. Your pet is not stupid, he just doesn't know what to do, he needs you to tell him.
Some people have set up macros to turn off the pet's fear ability, but I find if I just drag the pet spell down to the his ability bar that I can keep an eye on it and make sure that its off when I want it to be off, and on when I want it to be on.
Basic rule of thumb, if you are soloing and you want your pet to take the beating, then turn your pet's fear off, leave your growl on. If you are in an instance/raid, do the opposite. I know most people are thinking, duh. Well, I'm trying to state the obvious... just in case.
Now when do you want to use use passive/defensive/aggressive. Well, when you are in an instance you will almost always want your pet on passive. You get yourself into all sorts of trouble with your pet on defensive and especially on aggressive. If your goal is to wipe your instance/raid then by all means put your pet on aggressive. You're pet will get all excited an pull every mob in a two mile radius. When you realize your mishap your pet will come running all excited lolling his/her tongue all over the place (with about 20 mobs following). If you have your pet on defensive anything that attacks you with range will get your pets attention, usually also a bad idea. Best to be on passive for those situations. Now when I'm leveling I like to keep my pet on defensive so I don't have to tell him everything to do. And my pet reacts quicker than I do. Now there are only a few times that I would actually put my pet on aggressive. The best is if you are fighting rogues. Your pet can find rogues quicker than you can and he can help you sniff them out. Sometimes I leave them on aggressive if I want to clean out an area in a low zone while I'm farming. Oh, and also when I'm mad and want to cause a wipe for my raid (okay, I've never actually done that, but if I were mad I know how I could get everybody killed quickly, then you just feign death - this doesn't win friends or influence people, but if you are pissed at a pug it might be entertaining).
Well, that's all for now. Let me know if I missed anything.