If you want to keep breastfeeding your child, but you have to return to work, then expressing is the obvious answer. A portable electric or manual breast pump will be the easiest way to express at work, and you will also need to store the milk you have expressed to take it home.
Make sure that you take a cooler and a variety of storage containers with you to work in addition to your pump, and then store them in the fridge, or freeze them to take home with you. Breastmilk can be refrigerated for three to five days in the back of the fridge, where it is the coldest. It can also be frozen for between two weeks and six months. The former would be the case if you freeze it in a freezer compartment in a fridge, three months if your freezer section has a separate door, and six months in a chest freezer. Remember not to let frozen breast milk thaw if you are not going to use it within 24 hours (refrigerated) as it cannot be safely refrozen.