HELPLINE: 
Q: If I am declaring tax in France for the first time, can I do it online or do I need to submit a paper document?
A: If you are declaring tax in France for the first time, you cannot file an online return straight away. Your first declaration needs to be on paper, so the French tax authorities can set up your record.
This should be filed using Form n°2042, along with any relevant annexes, and sent to your local tax office (Service des Impôts des Particuliers). The administration will then issue your tax reference number known as the numéro fiscal.
You will usually need to include copies of ID, proof of address in France, details of your worldwide income for the year and your bank details for any refund or payment. Additional documents such as marriage certificates, children’s details or pension statements may be required.
Once your numéro fiscal has been issued, you can create an account on impots.gouv.fr and file online in all future years, which then becomes mandatory.