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.
See, that also depends on how much time you have, right
If you have more time and can accommodate a lot of freelance projects...that will earn you a lot
If you have less time, teach a limited number of students and charge a bit more as you get more and more credibility
You can work part-time for 20 hours a week on-campus or off-campus. And you don’t need a work visa to be able to do that! A student permit (aka student visa) is enough.
Read about the details related to studying and working in Canada here.