Thanks for writing. If you want it to grow as a single-trunked tree, you'll need to determine which stem you'll train to be the main trunk. Look for the strongest, straightest stem. Then cut out all others that come from the ground.
As your rose of Sharon grows, continue to remove any branches that form on the lower half of the trunk, keeping the trunk clear.
Eventually, your plant will get to the size you want --- then all you'll need to do is keep the trunk area clear of new branches and trim any leafy growth in the canopy when it gets out of hand.
The best time to prune your rose of Sharon is in late winter or early spring.
Thanks for writing. I hope this helps!
Senior Garden Editor, BHG.com
Better Homes and Gardens