The service area is represented by two lines. They are drawn right close to the center of the court. You have to stand in between two lines (service area) in order to serve the ball. Your back foot cannot step out of the line, when beginning the serve.
It has to be either on the line or inside the service box. Remember, when you finish your serve, you cannot step beyond the line instantly. Your front foot has to stay within the line.
And you cannot come out of this area, until the ball passes you and played or missed by your opponent. The yellow colored lines, in the following picture, represent the service box.