You can become ATC where you want (what region) you apply for training you take test you pass the test you go to Sweatbox training you get S1 you have CPT after some time you train more for Tower ATC (S2) after long trainings you get CPT and you get S2 raiting and so on
Once you have your account on vatsim, you have to go to your vatsim division and then contact to require assistence to your practice, then you will be able to realize your first exam and become an ATC!