I'm speaking from a mid 40's PLD perspective, so bear that in mind... ^^
Everyone will have their rotations that work for them, some are heavier on flash, some really juggle FB and SB combos and ignore RoH, some focus one mob, some cycle through, but the biggest thing I can tell the budding tank: position yourself to make your job easier. If you have a slept mob in the middle of two active targets you run the risk of breaking the sleep because of your target cycling. Drag mobs around, move them from where they are to where you want to be. Run through the group so you can see through the mobs to your healers and DDs and see what they're doing and if you've lost a mob. Set the targets up in a way that it's natural for the group to go 1, 2, 3, rinse and repeat. If you can't get them in an easy order or there's a pesky ranged mob, or one that likes to fire off Weaponskills and stray from the pack, that's when you have to communicate: sleep the Pieste, or the salamander (two that love to do WS), drag the melee mobs over to the sleeping ranged mob so you can transition easier.
Other fun things: when a mob is low (and you're a PLD), Shield Bash; they can't move, your DD can finish them off and you can start marking the next pull or continuing your rotation on the next mob. You don't have to combo all skills on one mob: hit the tertiary mob with FB, hit your primary with SB and RoH, or hit the secondary with SB and use the big RoH combo threat to hold the primary. Don't be afraid to Bash mobs that aren't doing anything, if you have a 6 second stun that's time they're not hitting you or doing AoE. Encourage your zealous DoT DDs to NOT stack DoTs on slept mobs, you can't touch them and spamming Flash isn't necessarily a great way to manage groups. Use your damage reduction skills liberally, not just as "OH SH*T" buttons--makes healers jobs easier.
Regardless, communicate with your team and be assertive at the get go. If someone has a better plan than you, let them tell you as much, but it's better to go in with something than nothing. And if you have any questions at all about PLD tanking (so far not yet to end game, but...) hit me up on Ultros (Brinna).
____________________________Ultros: Brinna Vahn