Undergraduates find their own jobs on campus or at non-profit organizations around town. Graduate students are hired by their departments in most cases.
Wages vary, but are never less than minimum wage.
In some cases the federal government pays your entire wage. They're paid every two weeks. You could earn the maximum stipend shown in the Handbook for Student Academic Appointees, as long as this doesn't exceed your financial need. Grad students are paid monthly. Sometimes, we can increase work-study awards.