ok, first things first look at getting a DECENT performance exhaust, there are loads of manufacturers that'll do em for a focus.

then look at getting yourself a decent air filter or inducion kit, again loads of different brands out there.

then you'll be wanting a "chip" (on your car it'll probably just be a remap) again LOADS of different places out there. (i'd personally reccomend www.eveolutionchips.co.uk)