A familiar is a animal that helps you and bonds with you during magick. It can be any kind of animal though a lot of Witches use cats or dogs. But it can be any pet you wish.
Basically, you grow with the animal, They will help guide you and help teach you. It will be a friend, a support and a teacher to you.
If you don't have a pet already that you're very connected with, you can adopt or buy one at a petstore or from a person. Or you can even take in a stray. My first Familiar that I had for about fourteen years was a stray cat that my older brother got for me from a house down the street where the woman was feeding many strays cats. He went and got one and brought her home for me. She was very scared at first, and even became rather defensive and a bit mean to pretty much everyone but for some reason she never was mean or defensive to me. As we grew (I was about six when I got her) she and I grew much closer. When I started doing rituals she became my familiar before I even knew what a familiar was. I always thought she was a witch in her past life or something because whenever I did a ritual she always wanted to sit with me in the circle. If she wasn't on my lap normally she would be sleeping on or around my altar or where I kept my crystals.
Once I learned what a familiar was, I started working with my Cat and asking her to help me with magick. She taught me a lot without verbally saying a word. I still miss her to this day, Goddess rest her soul.
Familiars are really wonderful to have, I advise you look for one. =)