How to fly with a 18 month old?

I am flying from Minneapolis to Orlando with my 18 month daughter, 9 year old son and a girlfriend. I am kinda confused as to how to travel with my 18 month old. Do I need to buy a separate seat for her? The last time I flew with her she was 5 months old and sat in my lap but now that she is 1.5 years, I don't know if the rules have changed? Anyone flown with the babies lately? Even if I do have to buy her a seat, does she sit in the actually seat or her car seat?