Help me be better!

[aodhan_ofinnegain]

I started tanking but got distracted but now I’m getting into it again I have a few questions.

At what point should I pop a defensive cooldown? And is there a particularly order like should I save the 30% until last, is there a common rotation or all depends on how many mobs you pull?

If you're pulling more than one pack of mobs, use sprint first before you've aggro'd if you have it available, as it allows you to outpace mobs so you take less autos as you move to the next pack.
Once you've got all the mobs together, pop Hallowed Ground first if two or more packs once you're about half health. If you have a WHM, that holy spams, it is worth waiting a bit as Holy is also an AoE stun so it would make Hallowed a little pointless.

When Hallowed isn't available, Sentinel > Rampart, weave in Sheltron as you can, once you're main cooldowns are on cooldown, make use of Reprisal and Arm's Length. If only a small single pack is all you can pick up, then just use light cooldowns.

Wall to wall pulling, is this a thing I should do in level 50 dungeons or wait until higher levels.

So far it’s going ok except the disaster hull breaker isle (bloody final sting).

Wall to wall pulls take some getting used to but 50 dungeons is where it becomes more expected, while levelling dungeons, wall to wall pulls can be less expected due to gear discrepancies, and typically levelling dungeon mobs hit harder compared to level cap dungeons, but there is plenty of opportunity to do so.

Should I spam aoe until there’s only 2 enemies left then go onto single target attacks.

Yes, 3 or more targets you Total Eclipse > Prominence until 2 mobs remain, then single target down the rest.
Once you reach level 72, you will get a spell called Holy Circle, which you will use on two mobs or more, for five uses while under the effect of Requiescat which you acquire at level 68, which boost your spells damage and healing by 50%, if you mp is above 80%. At level 80 you will swap the fifth Holy Circle for Confiteor, a heavy hitting AoE skill, only available under Requiescat. But don't worry too much about it until you get there.

Do you usually pull one pack and see how you get on and that determines how you will play for the rest of the dungeon?

Typically this is the worst way to gauge if a group is competent. If you pull one pack, the party wastes resources that could have been better used on more mobs, so the next pack has less resources to burn down. Honestly best advice I can give is go straight for the deep-end, and pull 2-3 packs, if you wipe you've learnt what the groups limits are, and you can adjust, but single packs are the worst means to test a groups capabilities.
You have to be used to wipes in this game they can happen often, and can be a good means to teach and help improve.

It is really difficult giving up control of your health and relying on others (I usually play healer)

Rule of thumb, only the last hp matters so don't panic if healers lets you get low, healers gotta DPS too. Typically I don't look at my health bar that often, try put a bit of trust in the healers.

Any other pointers would be grand I am focusing on paladin at the moment but will work through them all.

there isn't a whole lot that hasn't already been mentioned so far for your level, especially that doesn't also extend out to the other three tanks. I would say look up single rotations for PLD, it's pretty straightforward at early levels, but it does help to understand it.
There is a decent bit more to PLD at 80, but nothing that you can't ask for help once you get there, if you need it, but most importantly try have fun with it, and don't let a few bad runs discourage you from keeping at it.