I have three cats, and though I would dearly love for one of them to be a spiritual guide or helper for me, it is not to be. They are just plain and regular cats. Granted, one of them is freakishly smart (Charlie can turn keys to open locked cabinet doors!), but there is nothing otherworldly about any of them.
Though having a familiar is a common enough idea amongst witches and Wiccans (and other Pagans too), but it's not a requirement. I do not have a familiar, nor does anyone else that I know personally.
So what is a familiar?
The biggest misconception is that your familiar must be a cat, or even more stereotypically, a black cat. Not so. Any animal of any species can present itself as your familiar. In fact, the form your familiar chooses to take may be a message in itself, and may be completely unexpected.
Familiars can fill many different roles in your life, including that of teacher, friend, healer or assistant.
A familiar may arrive in the form of a family pet, but they should not be treated that simply. They are independent spirits that must be respected and honoured. You will develop a strong psychic bond with your new companion, and he/she/it will likely show a clear interest in your ritual work. I can't really go into too much detail on how a familiar might become part of your life, or how it may assist you with your spiritual growth. Each person's relationship with their familiar is special, unique and individual.
Of course, your familiar might not even be a physical being. Having an astral familiar is also a possibility. Astral familiars usually take the form of regular animals, but they also may take the form of more exotic beasts (such as a dragon, perhaps). Working with astral familiars usually takes place during dreams or meditation. But working on the astral plane is a topic for another time.
Where do you find one?
Sometimes you need to search out your familiar, but other times they will find you. You can't summon a familiar, only invite one. Meditation or scrying are two methods that may help you locate your animal companion.
They may arrive in your life when you call, or just when you are in need of them. And the flip side to that, is that they can also disappear from your life if you no longer need their help or guidance. I would say that most familiars stay with a person for a good many years, typically the life span of their animal hosts. But don't be surprised if they move on unexpectedly.
During the Burning Times, animal familiars played a sizable role in the prosecution of 'witches'. It was believed at the time that all witches had an animal companion, that was really a demon in physical form. Seeing an animal in relationship with an accused individual would often be considered evidence to their witchery.
Even something as meaningless as a mouse or fly in their jail cell. Even today, many non-Pagans who don't understand our beliefs consider any Pagan's animal companion to be an evil spirit in disguise.