Hello Lena,
If you are a european citizen arrived in France as a job seeker, you can not apply directly for the french social security.
You have to check first with the country you are from if you still covered by its social security system and if so, they will provide even an European Health Insurance Card (EHIC) or an S1 form.
If you are no longer covered by your country, you will have to subscribe a private insurance to cover your medical fees in France.
To know more about your social protection and steps with France Travail, I advise you to have a look at this webpage from the "centre des liaisons européennes et internationales de sécurité sociale (Cleiss)" : "You are coming to France to seek employment".
If you need more help, you can contact the CPAM’s English or French Advice Lines.
More information in the following topic: How to call the French Health Insurance Advice Lines?
Have a nice day.
