Its hard to answer your question with the limited information.  How old are you?  What division are you playing in?  What do you thinking you are lacking in playing your position? 

But basically a good WD shuts down the WA passes to slow down the opposition feeding into the circle, or turn the ball over.  She needs to be able to read play, have good footwork for fast changes of direction and dynamic drives.  They can grab the ball with 2 hands when intercepting, not just tipping the ball.  They have good vertical lift for intercepting.  

Although i haven't been able to give you anything that is directed at what YOU need to work on, I hope that will give you a start of where to look for drills for you to work on at home.