I'm a newbie today aswell
But I know exactly what you need to do to stop it happening
I had the exact same problem. with water in the passenger footwell after it p**seddown with rain
The simple thing to check is wether the bottom of your door card is wet? if so:
The problem is that the membrane inside the door is no longer stuck down properly
Perhaps you have had a window replaced or some internal door mechanism sorted recently, when somebody has had to take out the membrane to get to it. either that or it has come unstuck due to old age.
Any way to fix it, you will need to take the door card off, and dry the internals of the door off properly to allow it to be restuck down either with silicone sealant or using Duck tape.
If the membrane has been riped appart by some ham fisted mechanic then you will need to replace them with new plastic. I Used rubble sacks from B&Q as they were thicker, cut them to size by using the originals as a template and stuck them on with gaffer tape, and hey presto no more puddles!
I hope this is of some use as before i discovered this i spent a fortune on new door seals, window seals, seam sealer on the floor and spent ages checking all the seals etc when I came across the membrane loose inside the door card.
This was because I had a new window fitted and they didn't bother to stick it back!