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 residentresidents who were previously mentioned on their parents' tax return, not for expats obviously as the tax office wouldn't know about you).
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. To find it, you can enter your address on impots.gouv.fr under “contacter votre service local”.
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. Since you don't have a number yet, you have to check the first box and the tax office will then assign a new number for you and it will be mentioned 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.