The most important criteria in this process are excellence in teaching and ability to handle a course without supervision. Positions are awarded according to seniority. A memo announcing available positions and requesting applications is usually distributed in late January or early February for the following sessions of summer school. As little as you can afford. The ideal teaching load is one that does not compromise your effectiveness either as a student or a teacher.
Teaching is a time-consuming activity.
As a student, you are better off not teaching. Teaching more than that begins to cut into your studying time, particularly in your first year. Yes, and without any penalty or recrimination. You are not compelled to accept a heavier teaching load than you want or can effectively carry. It is not guaranteed.