Just work out the amps, watts/volts = amps, so say 120 watts divided 12 = 10 amps, this will be a continuous load, so buy cable that suits that.
Look on www.vehicleproducts.co.uk for the wire sizes/colours/prices.
Just tell them what the wires are powering and rating(watts, or amps) and they are very helpful.
You'll also be able to get your fuse holders, relays, termainls etc from there.
The formula is V/ I x R, V = volts, I = amps, R = Resistance.
tabetha