Well, at lower levels there's really not much of a toolbox yet. At level 15 you have what, Heroic Strike, Thunder Clap and Shield Slam?
Shield Slam is pretty much your bread and butter, it's very powerful single target threat and damage. Thunder Clap is good for a little bit of threat on groups, but you'll be struggling with larger groups for a while at those levels. The tools just aren't there yet. You're basically just using Shield Slam on cooldown and Heroic Strike when you have the rage. Thunder Clap to help with groups or for the debuff on bosses.
At 18 you'll get Sunder Armor which makes it a little easier to spread threat around multiple mobs or build quick threat on a single mob. Until then you're basically at the mercy of the Shield Slam cooldown and your damage output.