If you are solo-ing most of the time and need to tank a bit. A shield is good.
If you are in a team and the mobs never get to touch u, use watever that gives u the most buff.
e.g. Staff, versus 1 hand + shield, or 1 hand + offhand item.
So while leveling up, if u picked up a one hand thingy (whether main hand weapon, or off hand item/shield) that looks good but not as good as a two hand on its own, dont throw it away. You might find something to pair it up with that make them better as a whole later one :)