You're not going to be able to make a familiar by spell - nor at all for that matter. You don't just make them. They come to be; in cases may come to be one's familiar by a growing relationship/connection.
The familiar can (depending on if it's the physical or spiritual familiar) have various purposes. The physical familiar (which is really more a misconception than any thing) can aid in the giving of energy in a working. I don't care for the concept of a physical familiar so I leave it at that.
The spiritual familiar has many purposes such as, but not limited to, guidance in situations, protection, relaying messages, and help in astral projection/pathwalking/faring forth.