Im going to fly w my four month old, im so nervous...what should i bring & how r airlines w babies?

Do i have to pay for a seat for my son? Im flying southwest. What should I do for the popping of his ears? Someone told me to jst feed him while the plane is landing and taking off? Will he be sitting n his car seat....I mean where else woukd he sit lol Do u change ur babies diaper ryt there in his seat? Any advice/tips...just lay it on me!!! thank you