Combat support question?

in the Marine Corps. if i gualify for combat support field witch has many jobs in it like low altitude air defense gunner, and field artillery cannoneer, do i get to choose witch job i want in that field? or do they tell me what job to take in that field or if i choose one job, is that it? or can they make me switch? i want to know because i would like to asure my MOS in the corps when i enlist?