Do a soil test and fertilize after the results are in. Pay attention to the soil pH and to the level of secondary and micro nutrients (or lack there of). Mulch heavily (4" of any type of acidic mulch).

Phosphorus and pottasium move very slowly in the soil. If you apply some now, assume that it will reach the roots in about 6 months. I would apply some bone meal six months before normal leaf out occurs.

Hydrangeas require very little fertilization so one application in June (if you live in the north) or two applications in May and July (if you live in the South) should do it. Coffee grounds during the growth season are ok but stop all fertilizers in August-September. Try using cottonseed meal or manure. For a new small shrub, 1/2 to 1 cup is fine; you will need more for a bigger older bush.

Pruning is another story since aesthethics also come into play. You may want to hard prune the bush but I was wondering if you could post a picture so we all could see what you are seeing.