I used to assist quite a bit and have since hired and worked with many assistants and an old saying comes to mind: "you can train skills but you can't train attitude." as an assistant starting out I'd say having a good attitude is far more critical than having a particluar expertise.
Depends on the photographer and assignment of course, but being hyper-attentive to the needs of the photographer, learning quickly from mistakes, being willing to haul gear or hand-hold a redlector at the proper angle, knowing how to set up a light stand, use the basic functions of a flash meter and tape down cables would take you very far in my book.