Familiars are animals that are believed to assist magic users in spell castings, rituals, etc. Generally, speaking you could find an animal from a shelter, pet store, etc that could serve as your familiar. Others believe that you can't pick a familiar, the animal finds you.
I and most of my fellow coven members believe that it's best to have an animal that you feel a connection with be your familiar. However, you don't have to. The type of animal is entirely up to you.
Their presence is believed to increase the power of magical workings.
If you feel that you could benefit from having a familiar then by all means find one (and possibly give an animal a good/loving home in the process), but there is no law that says you have to have one.
The Witch Book: The Encyclopedia of Witchcraft, Wicca, and Neo-Paganism. - Written by Raymond Buckland.
Magic doesn't really have a color. It is the intent of the individual that determines whether the spell will be positive or negative. If you know your basics, generally speaking, you should be able to use magic, be it positive or negative, regardless of religion. I know a few individuals that don't follow a religious path and still practice magic.