Also, shoot on a monopod.
I have shot hundreds of field hockey games. I currently do it with a D7100 camera and a Nikon 70-300 lens set on speed 800 to 1000. I get excellent results. Both the camera and lens are not very expensive now. The Nikon 70-300 lens' automatic focus is quite fast, which you need for sports. You need the 70-300 range to cover action from close range to the far side of the field. By the way, I process the photos in Windows Live Photo Gallery, which does a very good job for most adjustments I need to make. Good Luck.
Taken on a stormy day in San Diego. D7100, 1/800, f/14, ISO 320