Thuja is the genus name for "Arborvitae" . Thuja is closely related to Chamaecyparis more commonly known as Hinoki falsecypress. Generally, I suggest reading everything you can bonsai related about Thuja, and Hinoki as cultivation techniques and training and styling techniques are nearly identical.
Key to remember, once a Thuja branch develops brown bark (no longer green) it will NEVER back bud in that region. Well never might be too strong, but you can not depend on Thuja to back bud anywhere brown bark has formed. This means, as much as possible keep some green everywhere on the tree you might possibly need a branch in the future. You can trim green branches short, but they must have green or that's it, no more green at that location. This is the single most important styling take away tip I can give you. Don't cut off branches completely, keep a little green until you are absolutely certain you won't be needing a branch at that location in the future.
Your tree is young, the main trunk is really skinny. I would keep as many branches as possible to try and bulk up the trunk diameter for a couple years. During that time read about Thuja and Hinoki, aka Chamaecyparis.