The fastest way is to buy a Radio Controll system, and in some cases thats the cheapest as well, depending on what your time is worth and what you need. They also have kits to remote controll a robot from a pc, via bluetooth, wifi, or regular radio controll bands. Making your own transmitter is pretty advanced, I'd only approach that if you have a good grasp of signal controllers and radio technology, and are willing to do some licencing, or at the very least checking some legal regulations.
By the way if you want to interface a hobby radio controll reciever with a PIC18 chip, I have code to do just that in my blog on this site.