Hi @4Coco,
I had a lumpectomy. The hospital breast nurse gave me a little donut type pillow which was great under the armpit. I also found one of those boomerang pillows helpful too because you can't sleep on that side for awhile. I didn't have much pain but was uncomfortable.
Shirts were great to wear. Also anything like a boobtube or firm camisoles were good to have a break from the surgical bras that hook up at the front. The hospital gave me a couple of those. You may need a dressing over any scar but depends where they cut you.
I slept ok the first night because of all the anaesthetic. After that it was a bit off and on because of the swelling and discomfort. After a couple of weeks it was easier and within the month I could sleep on that side.
Take it easy for a few days and then gentle walks help. The breast care nurse gave me little exercises to do to help recovery. Get a couple books of that appeals. Have a couple of meals ready. If you like music that's nice to listen to in hospital bring that if it helps you sleep or even a podcast or guided meditation.
Wishing you all the best.