Yeah Fhrx has the idea,
Buy the best stuff you can afford, even if you need to lay buy.
AMP, SPliTS and Sub. Buy them in that order.
I wouldn't recommend a wiring kit for a Skyline, you don't need half that shit.
You'll need RCA cables long enough to go from the front to the back. About a metre of BIG power wire and a Metre of BIG earth. (get the same size)
And you'll need speaker wire to run from the amp back to the front speakers.
Check out the other threads in here.