Your best bet is to contact your nearest French consulate in the US.
You can also go to the official french civil service website:
http://www.service-public.fr/.
I've copied some text from this site and here is a basic translation:
1) You are french by birth if one of your parents is a French citizen.
2) If you were born outside of France to a french parent, you can denounce/give up your french citizenship in the 6 months before & the 12 months following your 18th birthday.
1) "Enfant dont l'un des parents est français
Nationalité française par attribution
Vous êtes français de naissance, par "filiation", si au moins l'un de vos parents est français.
2) Si vous êtes né à l'étranger d'un seul parent français, vous pouvez répudier la nationalité française dans les six mois précédant votre majorité et les douze mois qui la suivent. Vous devez posséder une autre nationalité par filiation.
Pour toute information :
Il convient de s'adresser :
* en France, au tribunal d'instance dont dépend votre domicile,
* à l'étranger, au consulat de France de votre résidence."
Also check with your mother to see if she might have registered you at a French consulate where you were born (in your case, the US).
I have heard that this might help with the application process.
The first step is to get in touch with the closest french consulate to you. They should be able to answer your questions and get you started.
(The staff at the DC consulate have been very helpful to me so far):
http://www.info-france-usa.org/intheus/consulates.asp
Hope this info. Helps & Good luck!