You definitely want to get accustomed to using your breast pump before you go back to work — it can take a few tries to get the hang of it. But, there's no rush.
Focus on developing your breastfeeding relationship with your baby for the first few months. About a month before you go back to work, learn to use your pump and start storing some milk in the freezer. It's usually easiest to pump in the morning when you will typically have a good supply.