Breastfeeding

I'm getting ready to go back to work and I'm trying to begin storing my breast milk but I'm not sure how to begin. I just pumped after a feeding and put it in a bottle but do I place that bottle immediately in the fridge? Or can I keep it at room temperature to keep adding more milk from my sessions. I'm just not sure if I can keep adding warm milk to a cold bottle if I already put it in the fridge. Any breastfeeding moms have advice?