What is the reason for my antifreeze leak?

I have a 98 Ford Taurus, and recently it has been steadily dripping sizeable amounts of antifreeze. Originally it was only leaking water but now it is definitely antifreeze. Also, I turned on my car yesterday and while it was still in park (my foot was not on the gas) it was revving by itself even though it was not driving. What is the reason for all this and how much will it cost?