I hate it when tanks spend too much time marking (I main a Tank BTW). Macro "mk 1" to Tomahawk/Shield Lob, run in, use it. Boom done. You can mark the next mob without even actually using the ability. Just activate it when it's on cooldown (right after you use flash or an attack) and you'll put the #1 over its head without using any TP or messing up your rotations. Some people like fast runs. Don't spend 10 min marking mobs from 1 to 50 will get you all the commendations

On a similar note, trust your party, don't be afraid to run in. You're the tank, they're waiting on you to initiate. It's when you regularly play the "scaredy tank" that you'll see DPS or healers pulling.

Also, you don't have to flash repeatedly to keep hate for most things. Hate is pretty simple to maintain and if you just use an enmity generating ability like Flash or Overpower every couple of rotations, you'll usually be fine for most things.

Try to keep mobs with frontal cleaves facing away from the rest of the party. As a frontline role leading the party into dungeons, try to learn dungeons ahead of time unless you want to have that first time dungeon experience. There seems to be a much higher expectation on tanks to know dungeons and fights.

Remember your priorities are (in this order): 1) Get hate. 2) Survive. 3) Do more damage. When in doubt (Do I break up this combo to do Overpower? What piece of gear is better? for instance) always remember your priorities.

Other than that, I'll echo what some have said about trying low level dungeons (which are actually good exp now anyway), watching videos, reading forums for advice and so on. Practice makes perfect.

But above all, stick with it if you enjoy it. People can treat tanks really miserably sometimes and a good many fights depend on perfect positioning. It's easy to screw up and get called named for it. It's awful but it happens. Just keep your head up, ignore the naysayers, and to the best that you can. In the end, people will think you were born to tank and you can put all the other negative BS behind you.

Real life is a lot like that also. Just fight the good fight anyway. Haters gonna hate!