Hardwood flooring

I just housebroke two puppies, so I can finally rip up those smelly rugs and begin to refinish my hardwood floors. My husband and I ripped up the carpet last night and took the tack strip off as well as all of the staples. I just banged down all of the nails that were coming up. Now, I know I’m supposed to sand the floor down, but I honestly don’t know how. My hubby left me the electric sander and sandpaper which I figured out how to put together – but have no idea how to use it. I know you have to turn it on, that parts obvious, but I don’t know how much pressure to apply with the sander. There are stains of paint and who knows what to sand out. Also, there is some buckling in some spots from water damage. Can that be sanded down and finished like the rest of the floor, or do those planks have to be replaced? I’m a novice, so any advice would be helpful – any advice!! Thanks so much.