theres plent of posts on wiring amps but since you are new
there should be a remote lead on the headunit(should be blue)all this is is a live when the HU is on so if your HU unit doesnt have on often the electric aerial wire or even a seperate switched live could be used. but your pioneer will most likely have one.
as for the earth you want it as short as possible and onto the bodywork,if your mounting in the boot look for bolts into the chassis,seat bolts or as often used on the cinqs the fuel pump cover bolts under the rear seat.
make sure you are using decent heavy gauge cable as well