Well, I've never done anything like that myself. But I'd first find some recordings of how you want it to sound like, to use as reference. Then you need the audio program/software to add the "effects" etc, such as distorting your voice. Sony Vegas has some built-in sound fx options, for this purpose, but i haven't messed around with all the different options, since ive never done it myself.
One "shortcut" would be to maybe record your voice with your cellphone, then play it back near a mic and record it onto your pc. I've even heard people using walkitalkies to make their voices sound like they're talking through a radio. :P
I'm sure there are alot of different ways to achieve the effect, but for me, personally, I've never done it myself, so I'm guessing someone else here could help you with that, or fill you in on how to do it. And if not, you could try googling it. :)