FTM breastfeeding. Several questions

My little girl is already 3 days old 😩😩😩 we've been doing pretty good with breastfeeding but I have some questions/concerns.

It seems like I have real milk coming in now but still mixed with a little colostrum. How soon can I start pumping? Do I have to wait until it's more plain milk? When/how much should I pump. How to store it?

I have little bottles of supplements that the hospital gave me, but how little should I use them?Ive only used them twice so far. I was hurting so bad I couldn't hold her at all so I just told hubby to give her one:/

Sometimes when we're nursing I'll start cramping SOOOO bad and it makes my back start hurting to the point I can't even hold her up and I have to stop and lay us both down because it feels like my spine will break just from holding myself up. Is this my body sending me a sign that I'm doing it wrong??

How do I use it after I've stored it? How do I heat it for her?

Any advice/ tips for breastfeeding welcome!!!! ❤