The very basics are much the same as playing a DPS.

Only, you sit in your Defensive Stance while you do so and make sure to hit every enemy in an encounter at least once, preferably with a skill with Enmity bonus on it so it attacks you. Also, one of the very basic mechanics for Tanks is make sure that enemies are facing away from your party, because cleave attacks are the most common mechanics used for Tanks.

Outside of that, much of the rest of a fight is just spamming your rotations to deal damage to things.

Once you get higher level, there starts to be more optimization you can do, especially for Full Party content, such as tankswapping for particular mechanics, Circle-Shirking (As the Off-Tank, using a taunt such as Provoke and then immediately using Shirk on the other Tank to instantly just generate a bunch of Enmity for them) and dealing with Tankbusters with your CD's effectively.

Though, honestly, if you're leveling up from level 12, you'll pick up most of the basics during leveling if you're doing dungeons. Once you get high enough level for additional things to matter you can play around with your tools and also look up some guides.