It just occurred to me that what a lot of the new to magic folks need is a Magic 101 course. No offense to anyone, but such a course would be helpful. To answer your question muna, centering is usually done hand in hand with grounding. To ground yourself is to establish an energy connection to the earth, and to center that energy in your body.One exercise is to visualize a pole extending from the base of your spine into the center of the earth. Center yourself by aligning your body along its center of gravity. Breath from your center, feel the energy flow up from the earth and fill you. By doing this you are drawing on the earths vitality rather then depleting your own, and you are centered, in perfect balance. This is a basic technique of magical work, usually done before you work any kind of magic or ritual. There are many ways to learn to ground and center, this is one. If done correctly, you will feel the power centered in your body, and you will be ready for whatever the next stage of magic that you are doing.