Clothes confusion

Hi there. I'm a little confused about the various baby clothes and what's really necessary. I also don't understand all the different terminology. What do new babies wear at night and during the day? Is it different or not really? Lots of people have suggested gowns instead of onesies - the gowns are open at the bottom. Are these better for sleeping? I'm also unsure about how many I need of each age. I have looked up lists but find them confusing too. What is everyone buying? I'm having a boy (first child) due May 24. Thanks so much for your help.