One of the most popular misconceptions in tennis is the idea that you should deliberately "snap your wrist" on the serve. Trying to snap your wrist, rather than letting it whip forward naturally as a result of all of the energy you have built up from the larger parts of your body, can put your wrist motion out of sync with all those major forces and thereby damage both your serve and your arm. If you keep your wrist relaxed, it will do what it's supposed to without any deliberate effort on your part.
At this point in the serve swing, the racquet is headed toward the ball "edge-first," but an instant later (in the next photo), the forearm will have pronated so that the strings are facing much more forward. Let pronation happen naturally by keeping your arm relaxed and reaching up to full extension.
If you have tossed the ball slightly in front of yourself, as shown here, your weight will naturally transfer forward, too. Notice that the right foot is barely touching the ground at this point. As you develop a more advanced serve where you use your legs more, your legs will drive upward with enough force to lift both feet off the ground completely, but that's not something to strive for until you have a strong spin serve working quite well.
One other source of energy about to be delivered to the ball comes from the turning of the body from sideways at the start of the wind-up to now facing the net.
