Corky, I would not prune it this year if this is it's first season, I would just fertilize it according to directions. Next year, I would start trimming it.
But ONLY after the blossoms are done. And then do it right then so that you are doing it before it sets it's buds for the next blooming season. so do not wait on that, or you might ruin the amount of buds it will produce.
Ours were a bit on a straggly side when I came here, although as they had been in for years. Trimming them to the height you want will make them more compact. Now if you are growing them free-form under a tree, you only need to cut them to whatever height you like. But ours are up against the whole back of the house. So they get cut not only to a specific height, but also the faces (front side) is trimmed, to keep it lined up with the edging.