Social security & health
Social security registration
It's included in your work contract, but you have to choose between 2 institutions that will “follow your contract”: MGEN or CPAM.
Health insurance
The social security take care of the “basic” expense, but do not reimburse everything (glasses, …). Even if not mandatory, it is very highly recommended subscribing to a complementary health insurance (Complémentaire santée in French).
The MGEN provide one for ~30 €/month.