If it's your first time tanking, I highly recommend running dungeons like Halatali specificially. That dungeon has a ton of wandering mobs that can get caught in your tanking and it lets you get a better handle on it especially with facing the mobs away from the party. A big reason I also say Halatali over Satasha/Copperbell Mines is the the dungeon is also much shorter then either of those. So if you're leveling a tank it gives you a quick run through of how to tank and what you need to look out for. If you feel daring you can also pull more then just one set of mob there, but I would only do that when you properly manage aggro/use your cooldowns, and let your healer know you will.

But the absolute most important thing to do when tanking is let people know that you're still learning. Cannot stress that enough and most people will adjust to you if you're having trouble and offer you advice.