If you could tell me some of the areas of France that you plan on visiting i can help you out. In the meantime you could check out the pages on
French Hostels and hopefully this will help you when planning your trip. At 18 you will be able to stay in a hostel with no problem, depending on availability. I recommend that this will be the cheapest accomodation you will find which will mean you will have money to spend on other things. As far as saving up is concerned, this again, will depend on which part of France you want to visit. There are a lot of places where you can travel around and eat cheaply unlike in Paris where it can be quite expensive.
The
Flight Comparison Tool will also be very useful to you. This will compare flights from several airlines and then show you the cheapest fares available.
Also it is worth bearing in mind that there are cheaper months to travel to France and if you are looking to save money you could go on the months when the flights arent so expensive.