Great reply from Elaine. Wendy, just an extra bit of info here that might be helpful..
The box you need to tick says something like 'ce numéro ne m'a jamais été attribué'
The next box is then automatically filled in with a row of zeros.
This all works perfectly as long as you get the rest of the form right first time round. Because.. at the end you are given a username and password so that you can come back to your dossier. Except, if you have ticked this box, there is a bug in the system and you can't get back in. Urssaf then suggest you to start again, but then you can't start again with the same email address because the system says you've already made an application! Have had various conversations with the urssaf tech support people about this. They can reproduce the error but can't find what causes it or how to fix it.
So.. I suggest you go right through the form first to check that you have all the info required but don't validate it until you're sure that you have it all right. Ask here or elsewhere if there's anything you're unsure of.
If you do get it right, then usually everything goes quite smoothly after that, at least for professions libérales. There's another site now for other types of business.
Hope that helps
ps I would also think about leaving it until after 1st Jan.