Well, basically hit the enemies and get their attention before your party member does. Also keep the hate meter of all enemies filled on you (it's the bar below the class icon on the left of the party list name). Earlier on you can't do much besides running in headfirst and start whacking on each of them (at least get the two-hit combo in, that should last for a bit), but along the levels you'll get Flash which will increase the hate meter of all enemies around you. When you get this, make sure to apply at least once (twice if needed) to every mob you "pull". There is also shield lob which you get by doing the lv15 class quest, which you can use to "pull" distant enemies or those that you lost hate and decide to go after your party member.

I don't play a Paladin myself, though, so I'm not sure about the details later on.