Serve
The key to a good serve is having a relaxed hand and wrist, just like the motion used when throwing a ball. Therefore on the serve, the last three fingers need to be relaxed during contact in order to maximize use of the hand and wrist. To understand this concept, take a ball in your racquet hand; squeeze it as hard as you can and while squeezing try to throw it!? The tension in your hand makes throwing difficult. Yet many tennis players squeeze too hard with their racquet hand while serving and the result is a loss of power.
Simple Drills to Do Emphasizing the Last Three Fingers
Practice serving with the bottom one or two fingers on your hitting hand off the grip. Slide your hitting hand down the grip so the bottom one or two fingers are below the butt cap. This will give you the best feeling the role your fingers and wrist play in hitting a good serve.
Sometimes just practicing throwing balls is a good first step before putting the racquet in your hand.