I'm only a level 32 Paladin but so far I haven't had any wipes or problems Tanking in this game..

In terms for marking, maybe this will become quite important later on in endgame raids and such but currently I have never used them, well I did use them during my early levels but after finding out that damage dealers rarely follow the one's I mark I stop using it..

My personal advice for starter or new tanks in the game are;

1) Cycle through your Mobs..

2) Use your potions if the need rises..

3) Don't be an Tyrant..

4) Up-To date and Repaired Gear

5) Have fun

It's a game so just have fun!! Make friends and if S.E goes with their plan even get married.