First of all, welcome to the forum.

For a real micro entreprise, I think you only have to declare your earnings on the appropriate forms when doing income taxes. But you are limited to a very small income you can declare that way.

To register as a business, you need to go through the local Chambre de Commerce. They normally have a small business center designed to help new businesses handle the paperwork. Only issue is that most of the help available is in French only. But check with them. Some of the Chambres have people who speak English and may be able to help.

Registering a business is a royal pain - lots of paperwork, but if you can get someone to help you with the French, it's not that difficult to do it yourself.