Speedometer defective? Faulty transmission?

I have a 1996 Grand Plymouth Voyager SE. I’m clueless about car, except knowing something is wrong with mine. I have trouble shot all over the internet and before I sell my van for nothing because the transmission is shot, I would like to try and see if it’s the the speed sensor. I have had a fuel leek, whole in the compressor and I think this has caused the newest of my problems. I have had more than my share of transmissions just going out. I do understand that this van model is bad for having this problem. I really don’t think this may be this case.
The van will not down shift and the speedometer will not go back to 0. It remains In first at all Rpms problem is when trying to find any information on buying the part I really have no clue. My hunch correct?
Im a female and need help!