The tenant's rights
Take urgent measures: turn off the water if necessary, dry off... keep the damaged goods to prove your loss.
Immediately notify your landlord or the real estate agency that manages the property. If necessary, also inform the janitor, if the building has one, the neighbor if the damage seems to come from his place, or even the syndic of the co-ownership.
Declare the water damage to your insurance company: you can use an amicable report if your insurer makes it available to you. Send your claim by registered letter or give it to your insurer against a receipt. The claim must be declared within 5 days of the water damage.
On his side, the owner (or the real estate agency) will inform the syndic of co-ownership and the neighbor, if the property is in co-ownership and if the damage seems to come from the neighbor. He will declare the water damage to his own insurance.
Depending on the seriousness of the water damage, a search for leaks or an expertise may be necessary.