Centering and grounding is simple once you get the hang of it. It is an anchor for your mind and energy so that you aren't 'drifting' when you are practicing. It adds an element of stability. You can do this by focusing on your breath - inhale and exhale from your diaphram, rather than your chest. Feel the physical weight of your body and the floor your standing/sitting/laying on. Become aware from your stable standpoint.
The simplest form of a shield is the affirmation that you are safe. Think of an area that you feel is safe where no one can touch you, remember the feeling, and apply that feeling to the person, area, or object you're intending to shield. It will not be a physical shield that will stop things in their tracks, but it will prevent situations from that cause harm from happening. In more basic terms, it is a block of energy, so some side effects extend beyond preventing harm, though you'll get the hang of this once you research and practice more.