The heart of defense in field hockey is guarding your opponents and keeping the ball from them. Field hockey uses a blend of marking, which is like man-to-man defense, and covering, which is like zone defense. They often flow easily from one to the other.
Pick the opponent you are going to guard and move to the space where she goes.
Stand between your opponent and the goal in a spot that gives the most room for you to tackle, or intercept, the ball. Try to be ballside and goalside, which is between the ball and your opponent as well as between your opponent and the goal. If you miss the ball, you are still between the ball and the goal.
Watch the ball, not your attacker. You'll be less susceptible to falling for movement designed to distract you so she can move toward the goal.
Keep your weight on the balls of your feet. The more balanced you are, the faster you'll be able to respond to your opponent's changes in direction.
Be careful of using fake moves to distract your opponent; you may end up missing the play. Be patient and watch where the ball is going. When you have an opportunity, try to steal the ball or intercept a pass.
Use zone defense when you are not marking a specific player. Cover the space to the side and rear of your teammate so, if the attacker beats her, the marker, you become the marker until your teammate recovers. Think of it as a rotation.