How do I repair a leak on the underside of my car?

What is your question?

A week ago I noticed water bubbling and pooling on the floor behind my driver seat. The water was clear; it had been raining that day and I'm assuming water was being thrown up into my car. I was able to soak up the water and dry it up completely with a fan. While doing so, I noticed a depression in the floor where the water had been pooling. A cursory visual didn't reveal a hole but noticed that there is a spot just below where the water pooled is softer than it should be, not a hole. Can anyone tell me how water is getting into the car and what I need to do. I am a senior on a limited income. The car is in excellent mechanical condition and I really need to keep it running. Thank you.