Flying with baby

FarmWife 🌾 • 2015 👧🏼 2017 👼🏼👼🏼 2018 👧🏼 🌈 2020 👶🏼
We are planning a trip. Baby will be 6 months. How do we go about flying? Do we have to buy a seperste ticket? Can it sit in our lap instead..?