Derek’s first turn was Isaac. His first turn as alpha of his own choosing, of his own decision, was Isaac.
A broken, abused boy who didn’t ask for the bite, wasn’t forced the bite, but was given the opportunity to take it and made a decision.
Isaac is the first member of Derek’s pack, assuming Jackson’s bite didn’t take.
Erica will be the second, as far as we know from the previews. A dying young girl in the hospital, given the option to turn and be a member of something huge.
Derek’s choices for pack members are people who would want it, but not because they’re power hungry but because he can save them.