Unless things have changed recently, we just went along to a doctor and made appointment - nothing was said about registering - but then we are not full time residents.
You will find them to have plenty of time, very thorough and inexpensive!
The first thing to do is to register with a primary medical assurance provider. In most cases this will be the CPAM for your département, but if you are self-employed it will be one of many other organisations. Once you have completed that formality, you need to nominate a doctor as your médicin traitant, by filling in this form: http://www.ameli.fr/fileadmin/user_upload/formulaires/S3704.pdf, getting your chosen doctor to complete his/her section, and sending it to your health assurance provider. That only applies to French residents of course, if you are resident in Britain or any other European country and buying a holiday home, you go to any doctor and reclaim fees paid through your EHIC.
It's no big deal if you need a doctor before registering, you can go to any doctor but you will have to pay them and claim the standard proportion back later when you have registered. Even when you are registered, you can go to any doctor you like, but doctor fees, prescriptions etc are refunded at a lower rate if they do not originate from the médicin traitant.
Thoroughness, knowledge, bedside manner etc can vary considerably between individual doctors. It's impossible to generalise.
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