Automatic transmissions are very complex and not easy for just anyone to diagnose whats wrong with it or where its leaking. Best way to find the leaks are to jack the car up and put on jack stands for safety. Then use a degreaser or cleaner to remove all the dirt and grime from the transmission. Lower the vehicle and drive it at very slow speeds to keep from blowing the leaking fluid far from its source. Then put the car back on jack stands and look for the leak.
Common places for it to leak from is the fluid pan, filler pipe, cooler lines, or speedometer sensor. If its coming from the pan, most likely its going to be the gasket and just needs changing (its a good idea to change the filter while your in there if the car is equiped with it). Other parts, depending on where they are leaking, either will have to be replaced or tightend.
Some of the probable causes for the transmission to leak is that the transmission was over filled. If you over fill the transmission the fluid will find its way out through a seal or wherever because the transmission is not meant to be over filled, loose or broken input shaft bearing retainer or its o-ring or lip seal is damaged. If you just filled it up and its leaking, it could be due to over filling or not retightening the plug all the way.