Large leak evaporation emission control system?? (help!)

I have a 2000 GT and the service engine light came on so i took it to find out what it was and it said a large leak in the evaporation emission control system. I am a girl and i know some about mustangs but i have no idea what that means and i was wondering if anybody knew i dont want to take it to a shop and them just make up a bunch of crap cause they no i dont know what it is. PLEASE HELP ME if you can thanks