Marking

Now becoming a good tank takes practice. And it takes research. Because while other members of your group can go into the instance knowing what they can and can't do with their class, you will be in charge of pulls. This means that you should have researched ahead of time to find out which mobs (i.e. bad guys) can be crowd controlled (CC) and which ones can't.

This is very important as you don't want to ask your mage to sheep a target that can't be sheeped only to then have the mob attacking your cloth wearing friend. This can spell disaster because now you have a loose mob running around. Therefore, it is your job as tank, since you should be marking, to know which mobs are in the instance and how to mark them.

I recommend setting your marks up on your number pad. This way they are easy access and you can mark during battle. You can set this up in the keybindings menu. Mine look like

Numpad 1 - Skull
Numpad 2 - Red X
Numpad 3 - Green Triangle
Numpad 4 - Purple Diamond
Numpad 5 - Yellow Star
Numpad 6 - Moon (used for sheep)
etc.

Now the interesting thing to note is this is also the kill order that I use. I also use this to make a macro with kill order on it. This way I can quickly explain the kill order if I am grouping with new people. It looks like this:

/party Skull - kill first
/party Red X - kill second
/party Green Triangle - kill third
/party Purple diamond - kill fourth
/party Moon - sheep (if we have a mage)
/party Blue Square - trap (if we have a hunter)
/party Yellow Star - sap (if we have a rogue)
/party Orange Circle - shackle or mind control (if we have a priest)

This will cover most of your situations.

If the pull doesn't go according to plan, or if it is one of those pulls where you can only mark one or two of the mobs beforehand, be prepared to mark on the move. Always let your group know which mob you have targeted so they can be targeting the same one and not pull a secondary target off of you.

Also, research and understand how to do line of sight pulls. These can be very useful when you have a caster in the group you are pulling. You can pull around a corner and make that caster come to you. This way you are less likely to get adds (additional mobs) with your pull. And if you get feared, you should have pulled back to a place that you have already cleared so you don't have to worry about getting feared into an different group of mobs.

Overall, tanking is a lot of work. And there are a lot of people out there who don't do the research or put in the effort to learn the pulls, and it shows. If people find a good and competent tank, they will regularly invite you to run things and want to group with you. And this is the best compliment a tank can get. (Although sometimes, I would love time to do my dailies! :D)