Hello,
The criteria vary from school to school, and many schools won't offer you a GA position right away. That said, it would help if you have experience using a statistics software package and/or some other useful and relevant skill(s), and if you can make a personal connection with someone doing research that interests you who is looking for assistance. A good GRE and GPA are also among the criteria most schools use when determining awards upon admission. I don't know how your status as an international student affects things; you'd probably need to be eligible to work in the U.S. to get a funded GA position. Your best bet is to ask the school(s) to which you are applying for the specifics.
Good luck!
