Tanking is really simple, don't be afraid to make mistakes, use your skills, upgrade your gear. A well geared tank is less likely to get killed. Make sure you always have the best gear available for your level, you can buy most starting gear from vendors and get upgrades from dungeons. Recommended is to save the gear you gathered and use it as you level your second or 3rd tanking job. You can get them to say, 50 and sell all the gear, upgrade and continue the same path.
Start by pulling one group at a time untill your comfortable, if you feel you can take more, try 2 groups but use your defencive cooldowns, that increase damage reduction, parry and healing recieved. Also don't be afraid to spam your flash "PLD", overpower "WAR" or unleash "DRK". As you get practice you will get better at conserving your resources, if you spam alot you notice yourself how much you can do it and how to best grab the mobs.
Ranged mobs can be lured around corners since they need to stay in line of sight to fire at you. Simply get their attention and move around a corner, they will come to you in a nice pile to AOE.
I can also recommend you to watch what other tanks do while you dps or heal to get an idea of how they play. Or play the way you wish they had played, learn from their mistakes
For bosses, just turn them around and hold them on you, spam your aggro combo and use your second combo from time to time, at higher levels you can hit your aggro combo and then use your second combo rest of the fight, if the boss leaves you, just use provoke or throw at them, also, remember your the tank, so you can take more damage, even if you happen to stand in an aoe, you can survive, while dps and healer will take a ton of damage.
Next is stance dancing. If you pull 1 group you can start with defencive stance to get their attention and switch to attacking stance for more damage, same can be used for bosses, for multiple groups, stay in tank stance to make the life of healers easier.
All 3 tanks use difference resources, PLD tend to use mp alot. War uses TP and DRK uses mostly mp. Both PLD and DRK can return their resources, while WAR cannot. DPS need to help with gouds and other tp regens to help the war.
And thats the tactic i used for all the tanks, got em all to 70 but havn't done any endgame content yet. I got so good at tanking that i am hardly paying attention while running dungeons, mostly watching netflix and bingewatching series while tanking.