Congratulations on your pregnancy.
You may find this interesting reading;
kellymom.com/ages/newborn/bf-basics/importance-responsive-feeding/
The first few weeks of a babies life are hard work, and sometimes breastfeeding can be difficult to establish or painful. You shouldn’t expect it to be really painful (and it’s often quoted if it’s painful then something’s wrong) but the sucking sensation can feel rough and tender (mild pain that stops). Depending on your local area, you might have a lot of support and resources from La Leche, breastfeeding peer supporters, NHS IBLC’s or midwives. Being aware of how babies are at the start is a bit of a shock especially with cluster feeding and the frequency
kellymom.com/hot-topics/newborn-nursing/
The guidance mentioned by others not to express until 6 weeks is because your milk supply ( kellymom.com/hot-topics/milkproduction/ ) is considered established enough at 6 weeks. If you begin to pump during the first few weeks then your body will produce more but the pumps remove milk less effectively than baby. The risk is that you end up with an oversupply and must continue pumping in addition to feeding or risk blocked ducts, mastitis etc. For low supply, expressing in the early weeks might help build up enough for baby to EBF.
Pumping can be time consuming, you have the bottles and pumps to clean, bags or bottles to use / prepare. And it can be upsetting if you don’t pump enough to use as a full feed, often 1oz is all I could pump in an entire day (4 sessions). Some women can pump 4 oz in a few minutes, whilst others (like me) struggle. kellymom.com/bf/pumpingmoms/pumping/bf-links-pumps/
For any “missed feeds” provided by bottle, you will need to pump or potentially decrease your supply. It can take several days to restimulate your supply back up to meeting babies demand. The early morning and night pumping can be the most vital sessions to actual pump, seemingly (for me) is when I collect enough for a full feed.
Sorry this turned long, but I was convinced that everyone found breastfeeding easy and the shock of how hard it was for me was awful. Especially with my HV and Midwife telling me to just use formula, which I do but I wanted to continue breastfeeding too. Good luck with it all