Should I wait until I arrive to get the health pass or can I do it online before then?
Latest process requires you to convert your CDC card or equivalent at a pharmacy in France. Many folks on this form have done it recently and can give details.
The pass vaccinal cannot be completed on line. You must take your CDC card, assuming you are from the USA, and your passport to a pharmacy which will issue you QR codes what are your pass vaccinal for a fee of 36€. You may also scan these QR codes into the TousAntiCovid app which you should have downloaded onto your mobile phone.