Formal:
* Protégez-vous. (This is the most literal translation and is used in official or written contexts.)
* Prenez soin de vous. (This means "take care of yourself" and is a more polite and common way to express concern.)
Informal:
* Fais attention à toi. (This means "be careful" and is used with friends or family.)
* Prends soin de toi. (Similar to the formal version but more informal.)
* Méfie-toi. (This means "beware" and is used when cautioning someone about a specific danger.)
Specific situations:
* Protégez-vous du soleil. (Protect yourself from the sun.)
* Protégez-vous du froid. (Protect yourself from the cold.)
* Protégez-vous des moustiques. (Protect yourself from mosquitos.)
The best choice will depend on the specific situation and your relationship with the person you are talking to.