Center refers not to the center of your body, instead it is place within you that is the seat of what makes you, *you* and the place where all your energy is directed from and to. It is the source of your life-force, the seat of your soul. When your depressed you lose energy and when you are excited or you feel powerful, you gain energy.
To find your center, you need only become aware of it. One method that I recommend is feeling passion about something you love, like a piece of music. Bring this image into your mind and focus on your feeling for it. Focus inward past the emotion you have triggered to the actual source of the intence feeling. You should do this with your eyes closed. Continue to focus inward, past all thought and physical sensation, to the essence or 'base' of the emotion.