Just an extra information about marking targets.
Before lv 25-30, a lot of players will not care at all about your marks and hit whatever target they feel like hitting. You'll see healers pulling packs of mobs etc etc...
It can be very frustrating for you, but don't give up, keep marking and try to get AoE enmity and then to use enmity combos on multiple targets. This way you'll be used to do it for lv 30+ where it really starts to matter and where people will start to play correctly

I still see at higher levels tanks not marking anything: it's just horrible to play with them.