How to find the fuel cut off switch

Okay I have a 1995 Cadillac Eldorado my heater doesn't work in the car no cooling no heating I found a fuse underneath the hood it's a 40 amp I checked it and it was blown my when to replace it and as soon as I touch contact with the fuse in the middle terminals it popped it again seems like right after this the next morning and I haven't driven it yet since I popped at 5 go to start the car and the lights come on on the dashboard everything comes on except for it turning over the car won't turn over I thought it was the battery I check the battery it was fine somewhere in between this or something okay the cigarette lighter has been replaced somebody replaced it off the street I think he had the wiring mixed up because it never worked again so maybe that was tripping it I'm not sure I cut the wiring because your cigarette lighter in all the stuff that was hooked up I got rid of that and it's just bare wires not touching each other sure I unhook the battery and went to replace the main fuse to the heater and Cooling and unhook the battery this time just connected it put the fuse in in as soon as I went to hook up the positive to the terminal of the battery it Sparks big time I checked the fuse and it was popped again there's nothing else wiring wise that I had touched or done anything to so I'm just trying to figure out what in the heck is going on here somebody told me there's a shut off valve fuel shut off valve maybe to the ignition that's why it's not starting I'm not sure please help me