With all types of hydrangea, it pays dividends to build a strong bushy plant before you worry about flowers. The first few years after being planted are the most important for pruning, because it builds the foundation for years to come.
As a young plant, it is best to prune or pinch your plant in order to build a full bodied, well-branched plant. Every time you cut off the growing tip of a plant, you get twice as many branches and thus in the long-run, more flowers.
If the plant is leggy when you purchased it, shear the plant back hard by 1/3 to 1/2 its original size. Once it puts on an inch or two of growth, pinch the branch tips to remove just the growing tip. This tip controls branching. Once it is removed the buds below it will turn into two or more stems.
Once these new branches grow an inch or two, pinch the tip out again.
You can repeat this throughout the first growing season as you are tending your garden. Although you may sacrifice one year of bloom, this technique results in a well-branched, full-bodied plant that will have more flowers in subsequent years.
The second season in the ground, repeat the pinching practice (or lightly shear). Cease pruning and pinching to allow the flower buds to set.