I'm sure you have heard stories about familiars. Many date back to the Roman Empire. Most of those tales are of minor demons that aided their masters and mistresses in evil work. The reality is very different. In all cultures there are stories of magical animals, spirits or earthly, that aid shamans, medicine men and mystics. Those kinds of stories are far closer to the truth. Wiccans often have animal companions which keep them company. Whether they actually familiars that take part in working spells or conducting rituals is dependant on the Witch and familiar.
The purpose of a familiar can be many fold. A familiar can concentrate a Witch's energy or offer silent advice. Familiars are also company and can offer solace and healing. What your familiar will do, should you have one, is up to the God and Goddess.
Some Witches believe that it is not right to force and animal to participate in Magick. Others say that their companions choose to participate. I have a cat who watches over me as I practice my Craft but chooses not to join me. Each Witch must decide for him or herself where s/he stands in this matter. The calling of a familiar should only be done once a Witch has been Initiated. As a full Witch, he or she may then begin the process of acquiring an animal companion. The tradition you are part of may or may not have set rituals for calling a familiar. If it doesn't you can write your own.
Calling a familiar may be done with a formal ritual, where the God and Goddess are asked to send you a willing companion. Another method is to send out a psychic call. A mixture of both can even be done but keep your focus. After petitioning the God and Goddess, from within the Circle meditate while visualizing the need for a familiar. You might want to write a chant or set of words which state your purpose, to use as you meditate.
Once you have completed your call, end the ritual by saying that you will welcome any animal that the God and Goddess might send to you. Don't expect immediate results. It may take some time and come in an unexpected manner. A stray dog or cat at your doorstep or seeing the perfect kitten or puppy at a local pet store might be the familiar you've been waiting for. Even a wild bird that visits often or seems friendly could be your familiar.