Playing a Tank
Classes considered tanks by this article - Paladin, Shadow Knight and Warrior (generally any class with the ability to annoy monsters and take the hits can be considered a tank but the 3 classes stated tend to be the only ones with enough hit points and armor class to be a high level tank effectively).
NOTE: This article is aimed at those starting out, battle hard veterans need not read on!
Taking Damage
The first thing to overcome is to accept that your main job is not to deal damage (leave this to the monks, rogues, wizards of the world). Your job is to take the damage so the rest of the group can survive, pure and simple. If you are someone who always has the urge to smack things, try to remember that by allowing say.. a monk or rogue to tank they are going to cost the healer a lot more mana; meaning there is less to heal you and the rest of the party when in trouble (it can also reduce the number of beneficial spells, hit point and attack speed buffs for example, that can be cast on the party).
Building Aggro
When the fighting starts and you find yourself against more than one monster, even though they might all be attacking you at first remember that as soon as someone heals you or inflicts significant damage on a creature, all the ones you haven't really aggroed are going to run off to the healer/damage dealer. It usually pays to give the monsters a few good smacks first so that all the aggro is firmly on you, or at least keep a good watch on your parties hit points.
Assisting
While you are doing all the above, there is another thing to remember, assisting! While you are building up aggro and changing targets you aren't actually assisting anyone with the damage, but when you are sure all the aggro is on you it usually pays to all hit the same target. The faster each one dies, the less times you get hit and the less mana it costs the healer to keep you all alive. This is even more important when there is an enchanter in your group as if you are all fighting different mobs, they can't pick out targets to "sleep" as they'll be woken straight back up!
Leading
The tanks are usually the ones holding the party together (well other than a commanding enchanter), they will need to know their job and make sure everyone else is doing theirs. It doesn't really matter how many people are in your party, if everyone is doing their job you will still fight more effectively than the larger groups next to you. If you find yourself with a cleric who nukes (casts harmful spells) and an enchanter who sits idly then you can either try to tell them (politely) to start playing like their class or you can leave the party. Leaving may be the better option than having an argument and ending up dead (especially if the argument is with the only healer). This won't solve their problem but there are other people you can group with.
Fun
Doing all the above can be pretty hard to keep up with and you might wonder how any group survives but after awhile you will find yourself doing it all naturally. If your group is all working together and you are smashing everything you come across, you might even find it is pretty good fun!
If you do find yourself getting bored with it and the urge to do the damage won't go away or maybe you'd prefer to be the one healing, start up whatever you think would suit you, there is little point playing a class you find boring. There is nothing to stop you playing against the mold while solo of course but in a group you have an expected role and your party won't be happy if you neglect it.