Professors or other faculty in the specific field are usually the ones who do the hiring for GA positions-
The application process usually occurs a semester before the position begins, and students are hired based on their academic achievements, application materials, and faculty recommendations.
The first step for preparation is research. Go to the specific college department’s website to find out what professors are actively doing in their fields.
If you can find any information on the
graduate assistantships the department offers, read through that too. And use it during the interview.