diverter valve problem

I hope you can help me do this myself. My shower head had low pressure and water would run out the tup spourt. I have the kind that has a knob for the hot, the cold, and one to change the water from either the tub or the shower. I bought a book that said to replace the diverter stem. I did. Now I have very little water pressure out of the shower head, but at least the water no longer runs out the tub spout at the same time. I'm assuming that the hole in the diverter stem is not lining up properly, but when I turn the diverter stem to try to allow more water to come out the shower head, water leaks around the tub spout. What am I doing wrong? (This is my first attempt at fixing my own plumbing.)