Hi Denise,

You can learn more about the scientific method and making graphs for science fair projects here:
http://www.education.com/topic/help-child-with-science-fair-projects/

You'll find some graph examples in this article:
http://www.education.com/reference/article/Ref_How_Develop_Project/

For the Lung in the Bottle activity, if you had a way to measure the volume of air you are pumping into the balloon (such as through a bike tire pump gauge), you might graph the corresponding expansion of the balloon by each volume (you'd have to remove the bottom of the bottle though, to measure the diameter of the balloon for each burst of air). That's just one possible way to graph this. There may be others that come to mind for you and your child as you learn more about the scientific method and the science behind this activity.

Good luck - I hope all goes well with this science exploration!