You can't summon one, by all means. But normally a familiar appears to you, it sometimes is in dreams, words, or even in life itself.
Say you see a black crow outside your window, annd it doesn't move. You continue with life, and then you meditate...you see a familiar in your meditating state, it's a crow again. Finally you go to sleep and you see a crow in your dream. It's most likely your familiar.
I heard about this in a book, it said it's also a way to find your spirit guide.