You must sign a "bail".
If you don't, it means you have no contract, it's illegal.
A « bail » (plural is "baux") is the official document specifying how much you pay, what's the surface of your house. It's
also on your « bail » that is written for how long you rent. Usually, a bail lasts 3 years and is « tacite reconduction ».
« tacite reconduction » means that after the first three years, if don't want to leave and if you haven't received a notice from the agency or the owner, you can stay for a new « bail » : 3 years.