Shielding is the process of creating a magickal shield around yourself. It protects you from the energy of others, keeps you from losing your own energy, and/or prevents "psychic vampires" from stealing your energy. Shielding produces a shield of psychic (spiritual) energy around your aura (your personal energy which extends beyond your physical body). This way, others can't manipulate or steal your energy. You may think it's silly, but it happens more often than you know. If someone has ever given you the "creeps," or if after spending time with someone particular, you always feel tired and "drained," or have a particular mood, then those people have manipulated your energy. I had a friend for a short time that I practiced magick with. Every time I went over, I would become progressively more tired, and she would get progressively more lively. I didn't know it at the time, but she was a psychic vampire, which means that she stole some of my personal energy. She didn't know she was doing it - people often aren't aware that they are. Although some do it on purpose, and those are the ones to watch out for. No permanent damage was done, but needless to say I stopped wanting to spend time with her. Had one or both of us been more experienced witches at the time, we could have prevented the problem by shielding. Shielding yourself is also a way to prevent yourself from manipulating someone else's energy. The shield can contain any tendency you may have to try to change and/or take the energy of others, whether you do it consciously or not.
For beginners, the easiest way is to draw from the energy of the Earth. Center and ground yourself to the Earth. While your energies are still connected, imagine the Earth's energy rising up into you until you can't take any more. Separate yourself from the Earth and allow the earth energy to pass through your skin and settle comfortably around your aura. Do this until you have a layer of Earth energy surrounding your whole body with no gaps, holes, or seams. Use any remaining Earth energy to thicken your shield. It can be any size, shape, color, texture, scent, temperature, etc. that you want, but make sure it surrounds your entire body. Next visualize your shield turning from living energy into a solid barrier. See it thicken and slow, hardening while remaining pliant. When it's done, it should be like rubber - thick and hard, but buoyant. When you poke it, it should give way to your touch slightly. This elasticity ensures that anything that hits your shield will be bounced back off. Visualize your shield as reflecting these energies into the Universe rather than back at who sent them, otherwise you may end up breaking the "harm none" rule. Once the energies that you reflect are dispersed back into the Universal energy, they can be cleansed and recycled.
Another way to take care of energies that come into contact with your shield is by empowering your shield to dissolve and dissipate energies that come into contact with it, negating their intent and purifying them. If you create your shield with Earth energy, it shouldn't be difficult for it to do this - the Earth has the natural ability to cleanse, purify, and recycle energy.
For more advanced techniques and information, see shielding resources on the Links page.