All international students who are not citizens of the European Union, the European Economic Area or Switzerland must register with French Social Security when they arrive in France. This registration is free and can be done online on the official website etudiant-etranger.ameli.fr.
It allows you to get a Social Security number, then your Carte Vitale, which are essential to be reimbursed for your medical care, such as consultations, pharmacy costs and hospitalisation.
If you come from an EU/EEA country or Switzerland, you do not need to register: your European Health Insurance Card (EHIC) covers you for your healthcare in France.