Best way in the low level dungeons I found is to target the tank (figure out which function key they are; F2, F3, or F4) and then press T..."target your target's target" ( otherwise known as ASSIST). Then you DPS whatever the tank is focussed on. Tab works too but I find I lose focus really easily for some reason.
Hit Shift+T to target the first mob attacking you. You should maybe study your other keybinds as well.

That's if you're using keyboard and mouse, of course. If you're using a controller...you're on your own!!