Breast milk questions

So my baby is a week old today and I’m so confused about giving her breast milk that I’ve pumped. After research they say it isn’t good to give your baby milk that’s been sitting out for over 4 hours but my baby doesn’t drink that much when she’s feeding and i don’t want to waste the milk. They say you can refrigerate but then the milk is cold and they don’t want you to microwave which is the fastest and easiest for baby. Also i am using milk freezer bags, and want to know the quickest way to get it to be warm after i take it out the freezer. I just have so many questions and need guidance 😫 i don’t want to feed her bad milk or something spoiled. what do you guys do with pumped breast milk?