What abilities are you using? Paladins have some great defensive talents, although there is a few more buttons to press compared to other tanks. I'm curious what your rotation is now. Make SURE you have Righteous Fury on, it's hard to tank without it.
For pallys, your AOE rotation should be:
Hammer of the Righteous
Consecration (get Glyph of the Consecrator)
Use Avenger's Shield on CD as it hits hard and silences casters. Later, you will get the Grand Crusader ability allowing Avenger's Shield to come off of CD early.
Use Seal of Insight (extra passive healing) for now and then use Seal of Righteousness (more AOE) when you get it.
Use your holy power on Word of Glory for heals when you need it. Any other time use holy power on Shield of the Righteous for extra survivability and Bastion of Glory stacks for bigger Word of Glory heals.
Use Divine Protection to reduce magic damage.
Use Lay on Hands as a second life, when you know you are going to die.
ONLY use Divine Shield as a last resort. While it will protect you, it also drops your threat to 0. Meaning your healer will probably be getting punched in the face soon. You can taunt still so make sure to press it immediately after Divine Shield (and use some AOE for packs of mobs) to save your group.
Also, here's some general advice on tanking that may help:
First off, when tanking don't be afraid to run in to a group of mobs (within reason) you're the tank, you control the mobs, boss, etc. Your main responsibility is to keep the enemies off the group and sometimes making sure the bad stuff doesn't hit them by using your abilities, that's it.
Wait for the healer's mana if it's low. Don't be a douche tank that runs off miles ahead of the group into 50 mobs and die while the healer is drinking or goes oom. Try to keep an eye on your healer's mana bar and stop if it's about 20% or less. The healer is there to keep you (the tank) alive first and for most, so keep them close.
You may have noticed in low level groups DPS get impatient if the tank takes a seemingly long time to pull. You might be waiting for a CD, healer mana, or looking at a strat, or whatever. Don't worry about this, if the DPS pull, it's on them. You, as the tank, don't to save them in this case. Of course, you should do it anyway because DPS pulls are a good way to get a healer killed. But if DPS want to tank, let them, don't get overworked about it. Just keep the mobs off the heals. Low level dungeons are faceroll in heirlooms.
Lastly, try to look up fights so you know what to expect from the mechanics. When to use your CDs, where to move, what's bad and what's good. Tanks have to do this all the time, so get used to it now. Tanks are the lead dancers and should know all the steps to make the play successful.
Hope this helps!
TL;DR - Be a chill, shiny AOE machine.