Hi Miims,

The first step in becoming a student in the United States is obtaining a visa. Here's a website from the U.S. Department of State that is designed for international students seeking to study in the United States:  

http://www.educationusa.state.gov/
  
Their site includes info about getting a visa and financial assistance, as well as info about various study programs available to international students in the U.S.

Once you are residing in the United States and have a student visa (or other visa that enables you to reside in the United States legally), contact the local school district for your residence. You can research the district in SchoolFinder: http://www.education.com/schoolfinder/ (enter your city, state and zip code for the best results).

Typically, public schools in the United States are free to residents within the district, however, you should really speak directly with the district to learn its enrollment policies and procedures (particularly with regard to foreign citizens with legal U.S. residency).

There are private schools in the United States where French is spoken (along with English). There are even some public schools and programs that have French language immersion, such as this Wisconsin elementary school: http://www2.milwaukee.k12.wi.us/mfis/ They may be able to help you find a similar one for high school level. For private schools, tuition will vary, and some will also offer scholarships.

With regard to your question about college, I provided some college information for you on this page: http://www.education.com/question/study-usa/

Good luck with your study goals -- I hope you are able to achieve them!