Sometimes depending on the way the car is parked it can cause the water to pool in the door shut, and then it easily leaks past the seals, in which case just parking the car the other way round, usually nose down hill will prevent water pooling.
Other times it might be very obvious that you have a damaged seal as the rubber trim around the door opening could be cracked or have chunks missing. they do tend to get damage where the car is left out in the winter and someone wrenches a door open that had frozen shut you can tear the rubber seals that way.
another common issue, though I don't hear about it so much now, is they used to break into cars by bending the top of the door out. if this has ever happened to your car then the door might not be sealing properly.
At the moment you've got a big puddle and presumably don't know how long it has been there as you've not noticed it till now. So you need to get the interior dried out as best you can. then you can work on finding out how the water is getting in.
you can start with the obvious stuff such as changing the way the car is parked. checking around the door seal. then maybe get someone to run a hose over the car while you look for leaks inside. But when it gets to hiring smoke generators and such, then you might want to just get a garage to look at it for you.
