This is a very common question. A leaking transmission usually has one of the following three causes:
A crack in the oil pan gasket.
A cracked transmission case cover.
The transmission fluid has been contaminated by water, dirt, or debris.
If the leak is from the pan gasket, you can fix it with a sealant. If the leak is from the gear cover, you will need to replace it with a new one and reseal it with sealant. If the leak is from transmission fluid contamination, you should flush your transmission, fill it with fresh fluid, and install (or replace) a filter to remove any contaminants that could clog your system.