Flying on plane with 6 month old?

Well unfortunately my grandma has brain cancer which hurts me beyond belief. Well my question is I am flying airtran from ATL to Milwaukee WI and I know I can hold my infant free of charge which I will be doing but What do they do with the carseat/stroller and the playpen I need to bring? Do you think I will have to pay for it to be put under the plane? Also I have never flew on an airplane period so Im a little afraid but I need to do this some words of encouragement would be great also share your experiences with flying with your infant if you would like too. Thanks everyone for helping!