Have a look for ormi's project thread, or stav's - 2 of the most modified IS200s on here, and you can see how they went about it...
But modding is pretty individual, so where to start is different for everyone. For mine, it was because I had a wee bump and dented the standard front bumper. Found a TTE front bumper to replace it with, which was followed by a bunch of other stuff, including uprated ARBs, aftermarket stereo, TRD Side & Rear Skirts, 18" wheels, stainless steel dials and a bunch of other stuff...
Slippery slope modifying a car - ormi's thread is titled "the car I said I would keep standard" - should help you find it...