Decided to post some basic tips since I see a lot of the same issues with new tanks.
First, don't be afraid to tank. It's just melee DPS with defensives. if you can punch a rock, you can tank, at least in this game at this point
Second, every tank has an ability that secures aggro called a stance, like defiance, grit etc. This is vital to keeping aggro. Tanking without stance on is not just a bad idea, it's basically the only way to screw up aggro anymore. Set this somewhere that's not busy on your bars so you can pop it on at the start of a dungeon or raid, and then not accidentally click it off mid fight. You will never need it off in content until you are dealing with raids/extremes, and you don't want to deal with those as a new tank until you've gotten used to the role.
Third, Aoes are your best friend when pulling groups of enemies. 100 potency x3 enemies is 300 potency total. 150, 250, 350 is simply worth 250x3 over 3 attacks, 750. 3 100 potency aoes against 3 targes would have been 900 potency total for the same 3 GCD slots. This not only takes groups down faster overall, it keeps all that hate focused on you, and that's what tanking is about.
Fourth, you use your defensives in combat, not before going into combat. If you pop a bunch of them off and then rush in, you are generally just wasting time out of them and getting less out of them. Best way to use defensives is to weave them between GCDs, often at the start of a pull. Some pair well, and you can double those up(arm's length + rampart can be really nice in a big pull), but otherwise it's best to spread them out one at a time so you can get more mitigation over time.
Fifth, try to get all enemies on one side of your character. This isn't about mitigation, it just helps to group things more tightly so your dps and healer can nuke them all to oblivion at the same time without leaving too many stragglers you then have to fight after the main event. For larger pulls/enemies, circling the enemies can help keep them herded into a tight group. The tighter you can pack your enemies into a group, the more of them get hit by those aoes and the faster they all die.