Pumping and Breastfeeding process

Hi Ladies,

I am breastfeeding now using a nipple shield due to latching problem. I have heard about the breast pump and storage but not sure how I go about doing that bit. Baby currently wakes up every 2 hours to feed so when I do I pump as I don’t want to end up empty when he really needs some to drink. Was also wondering do I need to sterilise the pump every time I express milk or can wait for a few hours . Say I expressed milk about 11 am and stored around 30 ml and expressed again later around 3pm can I add the milks to a single container ?

When do I use this milk?