where can i find the mage stronghold quest?
and what is good about having a familiar?

You get the stronghold as soon as you finished the sphere quest and get outside, once you enter it again you receive your first orders.
Familars have certain different skills, but I actually never use them since I have people who can do better. In a fight you should never use, since when they get killed you loose one constitution point.
st.Schnupps