Are you talking about this one?
If so, you can't really summon a cat.
If you do want a familiar, this is my take on them: you can get an animal (from a pet store, a shelter, a friend, etc.) and do a spell to bond with this animal, to make them your familiar. Some Witches also believe that you can find an animal that you just naturally bond with, and that animal is your familiar.
You don't need a familiar to be a Witch, though.