Why would all the oil leak out of my car as soon as you crank it?

I have a 2001 Nissan Altima GXE 4 cyl. I went to work one day and at the last stop light before turning into the parking lot the oil light came on. I parked and when I got out there was a line of oil all the way from the road to my parkin. I looked under the car and there was oil everywhere. I had someone come tow it home. We've put oil it and there's no leak at all, or maybe a really slow leak, until you crank the car. Then every bit of the oil leaks out. I jacked it up and there are two puddles. A small one at the front of the oil pan and a very large one at the back of the oil pan. I can't actually see where's it coming from. I really have no idea where to start to try to fix it. I just need some idea of what this might cost before I have to pay someone to come tow it again. What are the possible reasons for the oil leaking out? Oh, and when you first crank it with it full of oil it ticks some, but once the oil is gone it ticks really loud. I was told I should also add that its a twin cam 16 valve 2.4L engine.