If it's the first time you file a tax return in France, you don't need any special code or number. It is however not possible to file online, unless you are between 20 and 25 and have received a letter from the tax office (that's intended for young French people who were previously mentioned on their parents' tax return, not expats obviously).
You have to get a paper form (or print it?) and send it by post to the tax office competent for your former place of residence.
There is a box at the very top of the form where you have to choose either “You are filing a tax return for the first time” or “You have already filed a tax return before” and then provide your FIP and fiscal numbers. The tax office will then assign a new number for you and they will be included on your “avis d'imposition”.
Beyond that, the documentation and forms are only available in French as far a I know and I don't think you can expect the tax office to provide any contact point for English speakers so it will be difficult to deal with all this without help.