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.