Halo:Reach commendation unlocking?

I'm sorry if this has been asked before. I cannot for the life of me get any commendation progress at all. I play on normal difficulty in campaign, with a Gold Live account, which I believe are the necessary things. Yet no matter how many headshots I pull off or how many grunts I kill, my progress for the commendations remains at zero - can someone tell me what I'm doing wrong? Thanks.