At that stage of life, your tree/shrub looks pretty normal. It will fill out on its own in time, but you would like to speed it along, there is nothing wrong with that. Minimally, to do this you can cut off the end bud on branches that you want to fill out. Cut just above the second bud. If there are two or more buds clustered at the end, treat them all as if they were all one bud. If it seems better (for structure or aesthetics) to prune deeper down the branch, that's not a problem. If the branch is so thick that there is no bud above which to make a cut, then prune down to where another branch diverges from the stem. Never leave a bare stick without a bud at the end.
This is what happened when said nurseryman cut the leader off....
http://garden.org/pics/2013-10...
there were originally four buds at the top, starting at essentially the same place at the top of the stem. The nurseryman removed the central leader (probably to propagate a new plant) and the remaining buds grew in a diverging pattern. If it were me, I would remove one of these three branches completely, as that odd branching configuration will become more prominent with time. Then, if it looks good to prune back some of the other two branches to make it look better or more balanced, do so.
I don't think Tara meant to pinch the top parts first and leave lower pinching for another time (like next year). This will produce a branching mophead at the top, with lower branches looking spindly in comparison. In time, this leafy "mop" will grow thickly and shade lower parts more, exasperating the problem and making lower branches even more spindly. Any parts of the plant that you want to encourage growth must have sun!
