What kind of creature can be a familiar?
Any animal or beast with Magical power, even if only a little bit.
How do I get a familiar?
Find the right animal (how is up to you) and develop a genuine bond with it. Any coercion, magical or mundane, will make such a bond impossible.
Okay, we’re friends, what now?
The initial bond requires one season of lab work, and expenditure of vis. The Arts involved depend on the specific creature – Animal and Vim are always appropriate, and others may be applicable depending on what the creature is and what it does. Larger creatures, and more powerful creatures, will require a higher lab total and a greater amount of vis.
When you bind the familiar, you also create three “cords” (which can be seen, in the literal sense, by someone with Second Sight) that tie you to the familiar in different ways. You decide on the strength of these cords during the binding ritual, depending on your lab total (the higher the lab total, the more points you have to distribute between them). Note that it is possible to spend a season strengthening the bond, and the cords, once the relevant lab total has improved.
So what does a familiar actually do for me, besides being a cool pet?
There are three main benefits, corresponding to the three cords mentioned above. The gold cord reduces your botch dice with spells and lab work, the silver cord grants a bonus to mental defenses, and the bronze cord grants a bonus to physical defenses. In addition, the magus and familiar both gain the True Friend virtue (which gives you bonuses when helping each other in significant ways), and can choose to use each other’s magic resistance rather than their own. Also, the familiar develops human-like intelligence if it didn’t already have it, and the ability to understand all languages that the magus can, and it will not die of old age as long as the magus lives. Familiars can also learn many abilities, including Magic Theory, so it is common for them to help their masters as lab assistants. Lastly, future rituals can give the familiar various powers that its species would not normally have.
How does the familiar work in play?
The book suggests a couple different options. One is that the magus’ player dictates the familiar’s actions, like a generic henchman (but presumably cooler). Another is for the GM to play it as a loyal NPC, or even for one of the other players to control it as a secondary character.