Installing a gfci contained by bathroom...? i have 2 black wires & 2 white wires coming out...

i have 2 black wires & 2 white wires coming out of wall, should i splice the 2 black & trim down them so i only own 1 black coming out of wall ( do the same w/ the white? )now, my gfci have 2 gold screws(hot) & 2 silver screws(not) also the gfci have 2 black wires coming offr the back of the outlet (jumping legs or somthing surrounded by that area.) not sure. by the road, it is the gfci w/ an on/off switch + a recepticle in one(can be covered next to a single decora cover) follow me so far?whoever wired this up the 1st time had it so that it turned on and rotten ,but i had no power when i plugged somthing within. anyone help near proper wiring
A/C fins disintegrating? Gray, crumbly mess.?
Answers:    in attendance is a phone # on the box. call that or ring an electrician before your wife become a widow.
Saving money on concrete Slab?
I think the 2 wires are the on- sour switch HOT wires for a light.On the GFI ,the 2 black hots to gold ingots ,seperate ,1 top,1 bottom.The white wires ,neutrals,1 top,1 bottom.